こんばんわ
as i understand it:
は is the topic marker particle; its used to either bring up a new topic which refers to the object coming before the topic marker or say that and object is smth. Like: A wa B desu
が is the subject/identifier particle; it's used to point out/identify a specific property of something or make smth clear
Generally:
if you already know the topic you are speaking about you use は
you use が when a situation is newly introduced
So the sentence would be correct:
もうひとつ上のサイズ
はありますか
however. if you want to ask if a specific thing is available/exists:
(item)
がありますか
Another example would be:
私
は学生。
As for me, (I am) student.
私
が学生。
I (am) the one (that is) student.
So in this case you use
が to distinguish yourself as t
he one beeing the student (maybe someone asked: who is the student?)
I hope i got this right ^^
If not please correct me.
additionally look here:
http://www.japanesepod101.com/2007/07/1 ... yle-you-2/you will find a really good explanation from Yuki 由紀せんせい in the comments