Hey Matt,
I've been working on a laser with an approximately 30" x 50" bed for the past 8 months. You can find my buildlog here:
http://www.buildlog.net/buildlog/view_log.php?id=254There is another guy on here (trwalters001) who is building a bigger machine than mine.
http://www.buildlog.net/buildlog/view_log.php?id=320I'd recommend reading through those pages first and seeing what two other people have done and some challenges that we've run into.
Challenges that come to mind for my machine were:
Selecting an extrusion that was stiff enough to do the job as well as being light ( I chose the 60mm x 90mm extrusion from Misumi)
Sourcing linear slides long enough to complete the project
Finding closed loop timing belts of the correct size to complete my z-lift drive
Trying to drive the y-axis with one rather than two motors (I chose to go with two)
Beam divergence of the laser at the far reaches of the table (the unfocused spot can get pretty big over an 8 foot distance
Building in enough adjustment points on the machine to properly level and square it
All of the structural components (slides, frame, enclosure, skins for the enclosure, etc...) will end up costing more
Having enough room to fit it in my apartment
Don't let this stuff scare you off though. It's definitely a fun project and you will probably learn a lot.