The text of the law is not hard to find:
Sec. 1.004. DISPLAY OF NATIONAL MOTTO. (a) A public elementary or
secondary school or an institution of higher education as defined by
Section 61.003 must display in a conspicuous place in each building of
the school or institution a durable poster or framed copy of the
United States national motto, "In God We Trust," if the poster or
framed copy meets the requirements of Subsection (b) and is:
So this law applies only to public schools, but it also applies to public secondary school and colleges, too.
(1) donated for display at the school or institution; or
(2) purchased from private donations and made available to the school
(b) A poster or framed copy of the national motto described by
(1) must contain a representation of the United States flag centered
under the national motto and a representation of the state flag; and
(2) may not depict any words, images, or other information other than
the representations listed in Subdivision (1).
There are no font size or colour requirements (except the implied requirement that the flags must be in red, white and blue (as specified in federal and state law) and that the writing be legible - again not explicit but understood and I'm sure that I can find case law that says that if a certain message is required to be written, it must be written in such a way that it can be read.)
There are no size limitations (again except the implicit "reasonable", since the courts have long understood that the law doesn't require one to be unreasonable)
(c) A public elementary or secondary school and an institution of
higher education may accept and use private donations for the purposes
of Subsection (a).
And those are the full list of the rules