I'd say shoot for however large you need to communicate what you want to say. That size is adequate for that map, I think.
There are file size limits listed on the attachments screen--if you're shooting for maximum detail, those are good guidelines. Generally speaking, most folks around here seem to prefer the PNG file format because it's compressible, not lossy, and permits transparency.
Now, on to the comments about the map! I like this style very much--it strikes a nice balance between photoreal and illustrative--artistic and yet neat and clear enough to be understood easily. I think it could use some more textural variety--something to "dirty up" the flagstones, make them look less manufactured, perhaps.
If you haven't already seen it, take a look at Torq's entry to September's Challenge: October Entry: Greenstone Monastery A few blood splatters, some soot, and light made a huge difference on that map.
I don't think you should go quite that far with it, but throwing some lighting variations on the floor, especially in the great hall where you can exploit those columns, could look really great. And dust collected in the corners--vampires aren't the world's greatest housekeepers, after all!
Good work, and I'm looking forward to seeing more of your work!