The scores on the parchment are called sirtut and are mandatory on a sefer Torah (SA OC 32:6) and mezuza but not on tefillin (Hilchot Mezuza 1:12). They help to write straight. It doesn't mean however that the letters have to be the same size. The Rambam in Hilchot Tefillin, Mezuzah and Sefer Torah does NOT mention that the letters have to be of the same size. The closest he gets (7:5) is to write that
One should not reduce the size of a letter in order to leave the
proper amount of space between one passage and another.
and in 7:9 he doesn't mention equal letter size as a requirement (same in 10:1)
All the above matters were mentioned only because this is the most
perfect way of performing the mitzvah. Should one, however, alter the
structure [of a scroll from that] mentioned above or not be precise
regarding the placement of the crowns, [the scroll is acceptable] if
all the letters were written as they should be.
[Similarly,] if one wrote the lines closer together, separated them
further, lengthened them, or shortened them, the scroll is acceptable,
provided one letter does not touch another, no letters are omitted,
extra letters are not added, the shape of even a single letter is not
altered, and the [form of the passages, whether] p'tuchah or s'tumah,
is not changed.
So equal size does not appear to be a requirement of a kosher Torah scroll and, in practice, they will sometimes not be the same size across a whole sefer Torah. However a more mehudar (beautiful) parchment will have as little difference as possible across letters.
R Reuvain Mendlowitz in his book Inside Stam explicitly mentions this (pp. 36-37) where he describes a sofer who changed his mind after writing 20 columns of a sefer Torah and wrote bigger letters for the rest of it as he felt they enhanced the beauty of the writing.
Experts who saw the Sefer Torah felt that this fell into the category of "normal" changes in letter size. Nevertheless the sofer felt that the change in letter size was not fitting for a high-quality Sefer Torah and, on his own initiative, offered to rewrite the first twenty columns (half a month's work) free of charge!
R Yerachmiel Askotzky in his book Tefillin and Mezuzos writes
(p. 135) Things which affect a pair of tefillin's aesthetic beauty
include ... writing that is not uniform in width, thickness, height or
Note that he speaks of beauty, not halachic permissibility.
Later (p. 136-137) he writes
If the writing is not in a straight, horizontal line, the parchment is
passul and irreparable according to all poskim. Letters that are written very slightly below the sirtut are acceptable. If there is
writing clearly below the sirtut, some poskim say it is still
kosher as long as the writing is straight. According to other
poskim, writing below the sirtut will make a parchment passul. [...] If the writing is wavy or on an angle, it is passul and irreparable.
That would make the "curvy rows" very challenging halachically but, of course, they should be shown to an expert before deciding anything.