What is the default icon size for macbook pro 13 inch?

The default size is not related to the Mac Model, it's related to the OS X version. For Snow Leopard it's 48x48 pixels.

For Lion and Mountain Lion it's 64x64.

You can choose the size that is right for you. I typically use either 32x32 or 36x36. Some prefer larger some prefer smaller. It's up to you.

I've always found the default sizes to be too big for my taste.

Off hand, I don't remember. Just create a new User and see what's presented as defaults. You can delete the user afterwards.