The default grid in Hammer and DeleD is 64x64; this will import to GoldenEye at 1.0 scale. Since the game supports scaling its maps, you can model at smaller (or larger) sizes and use some simple multiplication to get the scale you set your stage to use (for example, a map built on a grid of 32x32 can be imported at 0.5 scale, since that will double its size).
Having a sense of scale when you model your map is very important. Bond is about 2x2 on any grid (actually, a little smaller in third person, but 2x2 works well for the first person view), so your doors should be at least two units wide. Walls that are 5 to 5.5 units high make a good "normal" height ceiling, like the ones in Facility and Archives. Switch to a smaller grid size and zoom in for making smaller architecture, like steps, trim on walls, etc.
For those readers with a good sense of real-world scale, the units in GoldenEye are equal to one centimeter. This means a standard door in the game is 128 centimeters wide, or just over 4 feet.