As dapellegrini said, the big thing is the thickness of the Riccia. It doesn't matter how wide it is, I had mine carpeting the entire top of my 10 gallon tank (started from a small 1" square), but rather the thickness. I didn't trim it and the bottom 1" or so (it was about 3 1/2" thick) started to rot. It's quite difficult to trim once this happens so avoid waiting as long as I did. This was in a low light 10 gallon tank with a 17w NO light btw.
I wouldn't break it apart only because it will start to fray off and if you have a HOB
filter or other area of high flow it will go all over the tank and get sucked up in your filter and otherwise make a mess of the tank.