'Karl Forster' is an excellent and very popular selection. As with any plant, a larger one to begin with will be more mature than a smaller sized one and IME, purchasing locally will get you larger plants than ordering online or by mail. A gallon sized container (standard 6" nursery pot) will be large enough to reach blooming size this season and will only get larger (clump size) as it ages in the garden. Plant prices vary widely by region but here, a gallon sized common OG like Karl would set you back about $9-10, sometimes less.

The first ornamental grasses I ever bought about 6 years ago were in large 2-3 gallon pots. Got them at a local nursery. One was a Karl Foerster for about $20 and the other were large pots of Miscanthus 'Variegatus' and 'Adagio' for $30 apiece! They were expensive but they grew to a pretty decent size the first season and like you I wanted instant gratification.

The only problem was, the 2 Adagio were dead as a door nail next Spring. :( They were in a pretty cold and exposed location and apparently they weren't hardy to zone 5. Feeling a little sticker shock, I replaced them with 2 Gracillimus that weren't so expensive, and of course, weren't so large (but they are now huge).