OMG - reading this thread made me feel so much better! I thought I was the only one who couldn't grow R. rotundifolia and Java fern worth a darn!
I also like small plants because until a month ago, all of my tanks were 10 gallons or smaller. I just set up a 20-long and I find myself still avoiding the bigger plants. Besides, I guess I could be considered a "small package"
. Maybe a little "wider" than I care to be, but small just the same
So there was a time when Lobelia cardinalis 'small form' was my favorite plant. I still love this plant, but a few years ago, it suddenly melted on me (I thought it was a disease, but I now think potassium was somehow involved). Then M. umbrosum became my favorite. I still love this plant, too, but it seems to be a weak competitor for nutrients, and therefore, you have to maintain a somewhat high level of nitrates. Bacopa monnieri (thanks, Paul!) was nice because it was easy to keep and grew at moderate speed. For the past couple of years, Lysimachia nummularia 'aurea' has remained within the top three of my favorites.
Hey - notice that my favorites are all "small and round"... Like me!