How much space do I need to play with all the Arkham Horror expansions?
The main board is 2'x3' and each expansion board is 2'x1' (1/3 the size of the main board), which lends itself to two different configurations if you want to use all the boards.

2'x6' and 3'x4'

Also, as the second picture demonstrates, you can make clever use of business card holders to hold the cards using less space. Make sure too keep in mind that you'll also need to have enough space for each of the investigators.

When I've played with all the expansions at home I use two folding tables. I'll use a 6' folding table for the boards (It's about 2.5' wide, so can semi-comfortably fit investigators with items around one side if you push the board to the edge), and then I employ the use of a 4' folding table off to the side for all of the decks. This could easily be an endtable or whatever, I just happen to have a second, smaller folding table. The smaller table allows space for more investigators as well though. Which you'll need if you play with 10 investigators! (Since each expansion board after the first 'reduces' your effective investigator count, you can use 10 investigators without any of the limits breaking).

So yes, completely possible to  use all the expansions at the same time!