On the point of needing a bigger table I think you can just get creative with you battles. Very often players line every unit they have on a base line and throw them at the opponent. This is very unlikely to be what actually happened. You can always have 'off table' reserves. And if you need more battalions then have them recycle. There is nothing really wrong with having a 'fresh brigade' being feed on to the table that looks oddly like the one that just evaporated on the other flank.
I think GdA is a great game and gives the players a lot of interesting choices. One point I'd change in GdA is the strategic movement, it could do with some speeding up. Therefore, depending on how long you have to play. You might wish to deploy your front lines just outside of musket range. If you want to simulate taking long range Arty fire, it is easy to just roll those dice before you start.