Is it good gift for gf?

Yes, it's very flattering and cute.

How to choose the size？

I got my normal size and it fits great. There is stretch to these it has good wiggle room especially in the chest. If you have big boobs, i’d say go for a larger than normal

Is the choker around the neck detachable and how to attach the collar?

It’s completely separate piece and detachable. You can button it and put it over your head it stretches so it will fit.

Would this be flattering on very small boobs?

Yes I'm a B, and an unflattering B at that, and it hugged in all the right placss. My boyfriend wouldn't have normally said anything about it but he specifically mentioned it!

Can I put clothes in the washing machine?

I think it is recommended to wash by hand. I wash them by hand and they are good and quick dry. The metarial quality surprised me I thought it would rip pretty easy but it did not.