Here is an example of the original UI at 640x480:



Here is what things look like after you setup EasyTutu for 800x600:



EasyTutu at 1024x768:



EasyTutu at 1280x960:



Personally, I find that the 1280x960 is a bit too small, it sizes up the 640x480 by exactly 2x pixels so while it does retain the aspect of the original 640x480, it also turns out to be a bit blocky. The 800x600 UI is pretty close to the original, without showing dramatically more area. Either way, the EasyTutu UI is worlds above the default grey-on-grey of the original Baldur's Gate UI.

If you don't want to see wide areas of terrain, 800x600 is probably best. If you care a bit more about seeing lots of area, try 1024x768.