program size

Senario:

I'm an imac user, and I've downloaded program XX.
XX has many different sizes available from 1 to 800 000 kb.
The one I got to work was one of the biggest ones I could find.
I ran virus scans, and nothing came up.

Questions:

How do I know this is a full version?

Obviously a compressed file could be any size, depending on the compression program, but are there lists of file sizes available?

What are specific ways to look for infections/other programs within XX?

*email address removed*

here's a way to contact me, if you would rather not post. Any tricks/tips/answers would be appreciated

Thanks