A mono is a type of roof truss. The shape of the mono is basically a common truss that has been cut in half. Many monos of shorter length consist solely of a bottom chord and a top chord and have no webs or uprights whatsoever. Longer monos tend to include uprights and webs to distribute weight away from the heel joint, making the truss more stable.

In general, a mono is not expected to support much weight. Therefore, they employ smaller plates and are made of lower grade lumber than larger, more supportive trusses.

I have never seen a mono that was too large for a single person to build alone easily. Theoretically, they could exist, but that would have to be in excess of twenty feet long, which I can't imagine as a mono.

I think I should try to give you a better idea of what a mono will look like. If you read my writeup on trusses, you should be able to imagine a bottom chord and top chord doing their thing. Remember that a mono has only one top chord. The shorter ones will terminate here.
Longer ones, however will have webs. There will be an upright at the very end (point farthest from the heel) of the bottom chord, meeting the top chord at its highest point.

Even longer ones will have the same upright at the end and also an upright closer to the heel, around two feet from it. Then, a web will connect the two uprights, reaching from the top of one to the bottom of the other, depending where the truss is in the roof and how much weight it will be supporting.

As the monos get longer, the pattern generally continues, upright, web, upright, web, around every two feet. Some of the longer monos have a gable structure with only uprights.

On a more subjective note, monos are my second favorite truss to build. If you get a buddy to help you out, it's pretty fun. You'll usually have a lot of monos to build, so it takes a little while to do and they're darned easy to build.