We are finally in a place i hope to get to have my own arena, yay! However, we are still a little tight on space (we have just over 5 but all very usable acres). What would be the best arena dimensions for somewhat limited space? I plan to be riding and training (barrels, dry work) in it.

Depends on how many horses you have and how much pasture you want. I personally have 2 horses, a 3rd coming next spring & I would do an acre for myself. We currently have a 200x300ish at the barn where I keep them, it's also used as a dry lot turnout as well from time to time. Now that I'm used to that size, I can't go back. Before that, I had a 60x80 outdoor and 50x70 indoor and that sucked with a 16.1 hand horse. Ideally I would go 120x160/180 for just some light pattern work and general riding if you don't want a big one like we do. We have 10 acres and 16 horses 

At the moment we have 5, but only 2 are really rideable (one is trail horse sound only). But we have an area that we could easily use for an arena, but trying to decide how much to till up.