Translation:I want glasses that are a little bigger.
もう means "more" in this sentence. You want glasses that are a little more big. It's equal to the "-er" in "a little bigger." Without もう in the sentence, you would be saying that you want glasses that are a little big (implied for your head), which is a very weird thing to want.
The kanji spelling is 眼鏡 (眼 also means "eye", 鏡 means mirror), but etymologically speaking it should probably rather be 目金 "eye metal". Apparently the medieval Japanese coined their own word for "glasses" but rather than use the corresponding kanji they seem to have chosen to adapt the Chinese two-part kanji word as a whole (Chinese where 鏡 could also sometimes refer to lenses).