Turn off snap (lower right corner) when working, this allows you to place nodes right outside the map border, and let the functionality that restricts it to the map border kick in.

With snap on, you are basically just placing nodes at the edge, but not over it, so it treats it as a regular corner. Note that you can achieve this with snap on too, but you need to move your cursor much farther out from the edge of the map to hit the snap points on the outside and not those at the border itself.

Bring up one of the Schely trees for editing from the Symbol Manager, and then use Tools -> Distance to measure the height of it.

Now, edit your own symbol, and just scale it up so the height matches.

Note that your symbols should be edited in the source symbol catalog where you have them, if you edit the symbol from within a map where you have used it, only the copy stored locally in the map will be changed. Also, when you change it in the actual symbol catalog, be aware that maps that already use the original smaller version will continue to use it (because symbol definitions are copied from the catalog into the map on first use) unless you first purge it from the map.

I like the right one, but thinking about architecture, if you really want a stone floor up high, it is vital to get the stone supporting each other correctly, and that kind of require a more regular pattern and proper planning. Not really sure that those tiles there are the best option, but at least it is a planned pattern.

Also, a tower requires stones to be hoisted up by a lift, which means you only want the "right" stones up there in the first place. The right pattern is great when stones are resting on the ground, and you have a heap of random rocks to pull from, but up in a tower, you would probably only have the rectangular ones guaranteed to fit correctly together.

Oh, and to save you a click on the explode button, you can configure a symbol to automatically explode on placement, although this option is not compatible with the multi-sheet option for symbols.

For symbols, instead of copying from the symbol editing window, you can just place the symbol, then use :CC2EXPLODE: on it to turn it into it's basic entities.

You can also make symbols that place on multiple sheets: