Best grasses for a small garden?

Really depends on how small is a small garden ?
Zebra grass is Miscanthus sinensis, either the cultivars 'Strictus' or 'Zebrinus' both will reach a height of about 5-6 feet, 'Strictus is more upright and narrow, while 'Zebrinus' is more arching in habit, needs a little bit of space to look their best after several years,
Panicum virgatum cultivars are well behaved and will add a bit of height, also most varieties have good autumn colour.
For something shorter, you might try any of the Pennisetum alopecuroides cultivars. Molinia caerulea 'Variegata' and Hakonechloa macra 'Aureola' are good choices for smaller variegated grasses. Carex comans and all its forms are good, but tend to seed about a bit, Stipa arundinacea is beautiful grass, but unfortunately is a prolific seeder. Stipa tenuissima and Festuca glauca cultivars are excellent for dry sunny gardens but do need regular division about every three years to look their best.
I could go on and on.

I think it would be easier to tell you which grasses NOT to grow in a small garden. Most will be fine, but bare in mind that most like a sunny position, so if your garden is tiny and enclosed by fences/walls that might be something to keep in mind. (If you need advice about shade-loving grasses, then post another question!)

Don't plant Cortaderia or big Miscanthus such as 'Silberfeder' - these are big grasses. Don't plant Phalaris, as it spreads like wildfire. Bluespruce has mentioned some that self-seed, so they might not be suitable either. Other than that, there are loads of different grasses so just take your pick of hte ones you like the look of! Hope this helps... :-)

I've found the little blue Fescu grass has looked good in a small area of my front garden which is basically a gravel garden and I have tried to give it a Mediterranean look. It is a sunny spot too.

I've planted three different sorts of Carex - they are all lovely, evergreen and not tall. I agree about the Fescue - I have F.glauca 'Elijah Blue' and it is a lovely one! Have a look at your nearest Garden centre - the evergreen ones would give you winter interest as well. You have to cut back the deciduous ones in the early spring and they do tend to be tall on the whole.