Image size setting

Hello Richie,
thanks again for this great theme.
I have a question about images sizes.

As you know for the same post there are three sizes of the same image:
– in home one big rectangular for first post of category
– in home one small squared for second post of category
– in archive one rectangular for every post

In setting > Media I have three images size:
– thumb
– medium
– big

Can you tell me what size I have to set for each?
When I create a new post where I put the image and what size I have to choose?

Thanks in advance.