Height and width of the sutrah

It is mustahabb for the imam and the one who is praying alone
to pray facing a sutrah, because of the report narrated by Abu Dawood (598)
from Abu Sa‘eed al-Khudri (may Allah be pleased with him) who said: The
Messenger of Allah (blessings and peace of Allah be upon him) said: “When
one of you prays, let him pray facing a sutrah and let him draw close to
it.” Shaykh al-Albaani (may Allah have mercy on him) said: Its isnaad is
hasan saheeh. 

It says in al-Mawsoo‘ah al-Fiqhiyyah (24/177): It is
Sunnah for the worshipper, if he is praying alone or as an imam, to have a
sutrah in front of him that will prevent people from walking in front of
him, and to enable him to focus properly on the actions of the prayer. That
is because of the report narrated by Abu Sa‘eed al-Khudri (may Allah be
pleased with him), that the Prophet (blessings and peace of Allah be upon
him) said: “When one of you prays, let him pray facing a sutrah and let him
draw close to it and not let anyone pass in front of him.” And because he
(blessings and peace of Allah be upon him) said: “Let one of you use a
sutrah when praying, even if it is an arrow.” 

In the case of one who is praying behind an imam, it is not
mustahabb for him to use a sutrah according to scholarly consensus, because
the sutrah of the imam is the sutrah of the one who is praying behind him,
or because the imam is a sutrah for him. End quote. 

For more information please see the answer to question no.
117758 

Secondly: 

The Sunnah is for the worshipper to use something standing as
a sutrah, and it is better if it is the height of the back of a saddle or
more, because of the report narrated by Muslim (771) from ‘Aa’ishah (may
Allah be pleased with her) who said: The Messenger of Allah (blessings and
peace of Allah be upon him) was asked about the sutrah for one who is
praying and he said: “Like the back of a saddle.” Narrated by Muslim (771). 

Al-Nawawi (may Allah have mercy on him) said: In this hadeeth
it is recommended to have a sutrah in front of the one who is praying, and
it explains that the minimum sutrah is the back of a saddle, which is the
length of the forearm, or approximately two thirds of a cubit. This purpose
may be served by anything that he sets up in front of him.

End quote from Sharh Muslim by al-Nawawi, 4/216 

Ibn Qudaamah (may Allah have mercy on him) said: 

With regard to whether it should be thick or thin, there is
no definition that we are aware of. It may be thin like an arrow or a spear,
or it may be thick like a wall. The Prophet (blessings and peace of Allah be
upon him) used to use a short spear as a sutrah. Abu Sa‘eed said: We used to
use an arrow or a rock as a sutrah for prayer. And it was narrated from
Saburah that the Prophet (blessings and peace of Allah be upon him) said:
“Use a sutrah for prayer even if it is an arrow.” Narrated by al-Athram.
Al-Awzaa‘i said: An arrow or a whip is sufficient. Ahmad said: And what is
broader than that is preferable to me. That is because the words “even if it
is an arrow” indicate that something else is more appropriate than it.

End quote from al-Mughni, 2/38 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked
about the size of the sutrah for one who is praying. 

He replied: The best sutrah that a person puts when he prays
is the height of the back of a saddle, approximately two-thirds of a cubit,
but if it is less than that, it does not matter; even if it is an arrow or a
stick, it is acceptable.

End quote from Majmoo‘ Fataawa Ibn ‘Uthaymeen, 13/326 

Thirdly: 

The Sunnah is for the worshipper to stand close to his
sutrah, so that he can stop anyone who tries to walk in front of him,
because of the report narrated by Abu Dawood (695) from Sahl ibn Abi Hathmah
(may Allah be pleased with him) from the Prophet (blessings and peace of
Allah be upon him) who said: “When one of you prays facing a sutrah, let him
draw close to it so that the Shaytaan will not interrupt his prayer.”
Classed as hasan by Ibn ‘Abd al-Barr in al-Tamheed (4/195); classed
as saheeh by al-Nawawi in al-Majmoo ‘ (4/195) and by al-Albaani in Saheeh
Abi Dawood. 

The scholars (may Allah have mercy on them) differed
concerning the distance and from where it should be measured. 

Some of them said that the distance is three cubits from the
feet of the one who is praying, because the Prophet (blessings and peace of
Allah be upon him) prayed in the Ka‘bah with three cubits between him and
the wall. Narrated by al-Bukhaari (506). This is the view of the Hanafis,
Shaafa‘is and Hanbalis, and it is what may be understood from the words of
Maalik, because the distance between the one who is praying and the sutrah
should be as much as he needs to stand, bow and prostrate.  

See: al-Mawsoo‘ah al-Fiqhiyyah (24/184). 

Others were of the view that the distance should be
sufficient to allow a sheep to pass, from the place where the worshipper
prostrates, because of the report narrated by al-Bukhaari (474) and Muslim
(508) from Sahl ibn Sa‘d (may Allah be pleased with him) who said: Between
the place where the Messenger of Allaah (blessings and peace of Allah be
upon him) prayed and the wall there was a space where a sheep could pass. 

Al-Nawawi (may Allah have mercy on him) said: In the hadeeth,
Between the place where the Messenger of Allaah (blessings and peace of
Allah be upon him) prayed and the wall there was a space where a sheep could
pass, what is meant by the place where he prayed is the place where he
prostrated. This indicates that the Sunnah is for the one who is praying to
be close to his sutrah. End quote. 

Some scholars reconcile between the hadeeth of Ibn ‘Umar and
that of Sahl ibn Sa‘d (may Allah be pleased with them both) by interpreting
the hadeeth of Ibn ‘Umar, which mentions three cubits, as applying when the
worshipper is standing, and the hadeeth of Sahl, which mentions a space
where a sheep could pass, as applying when the worshipper is prostrating.
End quote. 

And Allah knows best.