MS Exchange questions

Hello 540964 and welcome,

The generic Email sizing information can be found here. Please see my comments below. Thanks.  

Does hosted Exchange Accounts share the same Domain and be private to my company (I have a domain that has to migrate) and does the hosted exchange accounts include public folders (shared folders that 2 coworkers can access)?

Individual Email accounts folders can be share and here is one  of many Internet locations that explains how to set this up.  I have personally tried this provided location (not that I am an IU Alum....lol) setup instructions and it works very nicely.  

Another issue: I understand hosted accounts Outlook 2010 provide 25GB per mailbox.  How much would extra space cost if it is needed?

I am not aware of any business class additional email box space that can be purchased to increase the 25GB size.