To merge cells in the editor, click in one cell, hold down the shift key and click in the other cell you want to merge with. Right click the mouse, select cell, select merge cells.
The html editor doesn’t give you control of font size specifically by px. It’s not hard to edit though. For example, if you want to set some text to 7.5px, then highlight that text and set it with xx-small. Click Save. Then look at the code generated and find your text. You should see html tags around it that look like this:
change it to look like this:
I’m not clear on where you want to set a margin, but you’ll have to edit that manually (I think … don’t find it in the editor). It’s a nice little editor, but not the most sophisticated out there.