Yes, they should turn yellow when they are ripe but I can only speak from my experience here in Cali--we generally have cool dry nights even in the summers and so citrus colors very nicely here. For most citrus, the best way to tell if something is ripe is to just test one. Meyers tend to be everbearing, with the heaviest crop ripening in the winter. The fruit can get quite large but that can be extremely variable and depends on a lot of factors. I just had some Meyers that were somewhat larger than an average grocery-store lemon, but I'd say that most Meyer lemons are average-lemon-sized.

They keep very well on the tree, and as they age on the tree the yellow will become more golden in hue and the lemon will get a bit juicier with a thinner peel that is quite smooth. When they are overripe, they will start to have a wrinkly skin but they're usually still pretty ok to use at that stage.

HTH.

Meyer Lemons can actually get an orange hue to their skin when ripe; I know mine do.