Make your map around
Height: 200
Width: 250
This gives you enough room to make enough continents and town, dungeons, etc.
Try not to make it bigger unless you run out of room.
I made my first project around 300x350 and I ended up making more places than I needed to simply because the continents looked bare to me.
So start at 200x250, and if you run out of space (Which doesn't sound too likely), then you can still add extra room by modifying the map options.
Also, if you want to link your towns and places, I recommend using roads and pavements
(flower trails and forests sound like a good idea as well).
I did this in VX, but I don't think '2k3' should be much different in this regard.