Upgrading to SSD

Hi.

I have a MacBook pro mid 2009 version and I want to upgrade it to an SSD. I have checked out a lot of options online for internal SSDs but I am not able to understand whether any internal SSD can fit into any laptop or are there some dimensions. I am looking at a 120 GB SSD at the moment.

Also, do macs require any special kind of internal SSD?

Thanks!