for their housing, they don't really need much more than a three-sided shelter. Make it as large as you would like. It should be tall enough for you to walk into, of course, but the other dimensions should be around 8 x 8 ft or so.

If you plan to build it yourself, you will need some tools. Use a post-hole digger to dig the base for your corner posts. All four holes should be at least 2-3ft deep. The placement of these will also define your enclosure's dimensions. Once the holes are dug, place the posts, (maybe 10ft tall?) into the holes. As you place each post, fill the hole with cement and prop up the post until the cement has begun to set. Use a lever to make sure the posts are perfectly vertical. After that, you just buy some lumber and basically start building your walls, and then a roof, around the posts. To have a slanted roof, set the "front" posts slightly higher than the "back" posts to give your roof a natural slant for drainage.

If it's a properly built enclosure, then no. There will be no noticeable SQ difference between the two so long as they have the same internal volume and an equal amount of strength to prevent flexing.

You could put a glass window at the lowest point and look at that, either visually or with a beam set at a glancing angle and going to a photosensor. You could also put an glass plate in there and deposit two electrodes on the glass. Measure the current between the electrodes and it will increase dramatically with moisture in there.