Yes, you can change the allocation of space for each mail box independently. Log into the control panel and click the "Mail" icon to get to the mail settings. In the list of mail boxes, you will notice that the allocated space is Blue. Click that Blue (it's a hyperlink) it will lead you to a page where you can change the space allocation.
It is best to keep the mailboxes as empty as possible, so at the risk of sounding like Bill Gates, 2 meg ought to be enough for anyone.
The biggest file attachment size would be roughly 7/8ths the mailbox size. If you are sending files larger than 2 meg, you are using email for the wrong purpose. I'd go so far as to say larger than 1 meg is too large. There are several open source applications that allow your web site to be your source for file sharing. Also, with the advent of Microsoft Office Groove, there is no excuse (other than not wanting to buy Groove, which is a lame excuse for a business) for using email as your file transfer mechanism.
Let me conclude by telling you a True story that happened to one of my other clients about 4 years ago. You may remember the so big worm? If not, here's what it did. So big was a worm that sent itself via email to everyone in your outlook mail folder. Not terrible in and of itself, but it also attached a 5Meg file with the email! Overnight, one of my clients used up 20 Gig of bandwidth. We had to turn off his mail service to prevent him from owing us more than he was expecting. He wasn't happy about having the mail shut off, but he was happier about that than the fact that he had this worm that was running up his hosting bill.
Since that time, most hosting companies have throttled attachments below 5 meg. And, I personally set my mail box at 2 meg just so my friends (and enemies) CAN'T fill up my mailbox with useless images and other attachments that they could just as easily have sent me a link to.
It's your space, you can use it how you want. But, my professional recommendation is to keep the size as close to 2meg as is reasonable for your business. That is, if you get more than 2 meg of legitimate email over night, yeah, I guess you'll need more space. If it's just for doing file transfers. I strongly urge you to find an alternative way of sending files back and forth as fast as you can.