Yeah, in such a small tank dwarf sag is not really going to look like a tradition carpet (or lawn) but really just a foreground plant (or even a background plant depending on how tall it grows). In my lowtech 10g it typically grows between 5-7 inches tall, although as Fresh already pointed out it's height varies under different conditions.
The addition of CO2 (pressurized or DIY) would give you quite a few more options. Ranalisma rostrata and Micrantherum umbrosum "Monte Carlo" are relatively easy medium light/CO2 supplemented carpet plants once given time to establish. Staurogyne repens is a popular stem plant that can be trimmed and replanted to look like a carpet too and does fine in medium light. Various mosses (flame, fissidens, mini pellia, Xmas, peacock, etc) tied to SS mesh or stones also can work very well when done right.
If you decide to keep it low tech you can do the m. minuta (although it's a slowwww grower in low tech) or maybe just look into doing a bunch of crypt parva as an easy grassy-looking foreground plant.
