I did that a few years ago, and I made the base for my 4X5 Beseler slightly larger than standard, but not much. A lot of things enter into the design. First you want to be able to position your easel to print from any portion of the negative. If you do a lot of high magnification cropping, this can be a problem; but if you usually print nearly full frame, you don't need much more than the standard size. Next, how big a print are you likely to make? You need a base at least big enough to position the easel for both horizontal and vertical enlargements of that size, with enough wiggle room to align the scene. It you limit yourself to 16X20, this is easy enough; but if you make 24 X 30s, you may have a problem. Along with that consider the limits of you enlarger - what size range will it handle with your setup and lenses. No need to build anything bigger than you can handle. Consider the enlarger mounting. Will it be attached to the wall or to the baseboard. Attaching it to the wall can give you some extra versatility when it comes to making the baseboard, but you will be stuck with that enlarger position. If you mount to the baseboard, you must consider the mounting and bracing. Last remember that you must be able to reach all the enlarger controls when it is at maximum height.