Sizing question about shoes for an infant

When you buy shoes for your DC, what size do you buy? Is it like clothes where you buy it a little bigger?

My DS is 4 months old but his clothes is 6 months in Carter's and 3-6 months in Gap (but he's about to outgrow these).

I bought him a pair of sandals at the Gap and got size 6-12 months. Now I'm wondering if these will be too big. He'll be about 7 months in the spring.

TIA! I'm obviously a new mom

How about socks? Do you buy these in a bigger size too?