One way you could approach it is to decide the size of the average house. A medieval or earlier house for commoners were usually a tiny affair, often no bigger than my living room. Then, based on your house size, so you can used that against the sketch to determine size.

Lurkers are always welcome too, but don't hesitate to jump in if you have any questions or a map to show off or get criticism on.

With the exception of Symbol Set 1 & 2, you'll find the entire list in the Tools menu -> Add Ons

Only way to check SS1 and 2 is to go into the new map wizard, and under overland and dungeon respectively, see if you have templates for these.

That CAD Blocks site has a lot of good stuff. I've added it to the list in the Free Symbols resource thread.

These CAD blocks are actually ready to use symbol catalogs, which is very neat. I first thought I would have to resort to just copy/paste from the drawing, but load them up as a symbol catalog, and they show up as proper symbols.

I'll probably never to cities as competions. The most common thing I hear every time someone announce a challenge or competition is "sorry, I just don't have time right now, working on this other map/real life/etc...". And cities are one of the most time-consuming things out there, which certainly wouldn't help with people having time to take part. (And while you can do something like a Watabou city really quick, that wouldn't be much of a mapping challenge considering most of the work is just changing properties on the import, especially when all the entries are like that)

And for that two-parter idea to work, I would have to put really strict rules into place regarding how to build the city for part 1 for it to be suitable, I don't think that is really going to work out well.