I can see how confusing this must seem to a 'newbie.'
I like a shorter ruler for cutting smaller pieces and the longer ruler for strip cutting or larger pieces. 6" z 24" fore my longer ruler.
Brands: Omnigrid or Creative Grids. With Creative Grids the rulers don't slide or slip. However, I also love the nifty Notions 7" x 14" and use this a lot too.
The main take away is: whatever ruler you start with, keep using it for the whole project for that sized pieces. You may use 2 rulers: a small one and the larger one ... whatever ... those are what you will use throughout. Mark them if necessary with painter's tape. If you interchange your rulers you may end up with different sized pieces.
ali