I would always hike up the size from now on - as big as I can get away with. More often that not I make every painting at least A3 imperial - 5000 pixels because - yes sharper details, but also, you never know when you'll want a bigger version or a crop etc. I'm currently working on a personal thing which is about 7000px and Artrage is handling it fine on my iMac. Interface operations like select, undo, move layer etc are a bit slow whereas in Photoshop they would still be instant at this size, but they're different beasts and I'm always hopeful the fantabulous Ambient team will find ways to speed that stuff up. Besides, there are about 36 layers in that file so if I flattened it things would be quicker.
Cheers