Possessive determiners in French
Possessive determiners, often called "possessive adjectives" in more traditional grammars, are words like my, his, our in English. They introdce a noun phrase and express who 'owns' it.
In English, the choice of possessive determiner sometimes depends on the gender of the owner: his indicates that the owner is male, whilst her indicates that the owner is female. In English, the choice of possessive determiner doesn't depend on whether the 'thing being owned' is singular or plural. (That is, you can say his book or his books.)
In French, things are slightly different:
Table of possessive determiners ("possessive adjectives") in French
Bearing these points in mind, here are the possessive determiners in French. The first column contains the French subject that would represent the owner. In the second column are the corresponding possessive determiners, depending on whether the noun that they specify is masculine, feminine or plural. (There's a slight complication with feminine nouns or adjectives beginning with a vowel that we'll see below.)
The forms ma, ta, sa before a vowel
The words ma, ta and sa, are replaced by the corresponding masculine form (mon, ton, son) if the following word begins with a vowel.
Suggest a change / proposez une modification
In this case, it doesn't matter that assiette starts with a vowel, because the word immediately after ma is grande, which doesn't start with a vowel.