I use Water Lettuce. In most tanks it will stay about 2: across. In high light CO2 tanks a littler bigger.
In my tanks I can monitor the tank just by watching how fast it grows and how long the roots are. I like the roots in tanks were you want a darker, funky kinda of look.My beta loves to hang under the roots. They are great nutrient mops and help shade a tank if it's too bright. Super easy to grow. I've got enough extra to fill a 10gl extra right now, PM me if you want some.
Duckweed is an aquatic social desease. It grows insanely fast, when dries on tank glass or filters you need to scrape it off. I have it in the lake near my house, I sell it to Goldfish keepers. Goldfish love it.