Hi Kris, the world is your oyster if you are going custom. Do you want glass or plastic?
There is quite a buzz about an Alberta based company called Inter-American (glass). They build custom tanks that are gorgeous but there are some reports of poor customer relations and missing delivery dates by substantial amounts of time. J&L Aquatics can give you a quote on both glass and acrylic tanks made to your specs.
As far as what size to go, the only limitation is money. If I were going custom, my dream would be an 8'L x 36"W x 30"H for a total of ~450gal . If you are a planning a reef and are going with custom dimensions give the front to back size some consideration.
All of your questions depend on variables such as what size you really want, what you want to keep and what your budget is. Have a look at
www.jlaquatics.com . They have all their pricing listed so you can do a quick cost projection.