Hi there... nice to meet you!
You get 3MB for free. If you pay to upgrade your membership, the maximum file size increases to 4MB, 5MB, or 7MB (depending on how much you spend). I'm pretty sure it costs $50/year for 4MB (be sure to sign up for "quarterly" payments as opposed to "monthly" payments, as it's cheaper that way).
I've noticed the sound quality can be very good at 4MB or less, whereas going under 3MB is stretching it a bit. However, I've heard some songs subbed at under 3MB that sound fantastic, too... it probably depends on what kind of equipment you're using.
I'm currently paying $100/year for a 5MB size limit. But honestly, I could tell very little difference in sound quality beween a 4MB & a 5MB size file. It's funny... YouTube and Facebook give you upwards of 100MB for free!
Mark