Anemones that are refered to as Saddleback anemones are usually haddoni carpets. They can grow as large as 24" across, but usually top out at about 18".
They can and will eat the other fish in your tank and not all clowns are fond of them.
They are not as difficult as gigantea carpets, but a little more difficult than BTAs to maintain.
There are a lot of blue gigantea carpets out at stores right now. Make sure that it is a haddoni carpet. Giganteas require a special set-up.
Look for pictures in a clownfish/anemone book or in the FAQ up on the top of the list to tell the difference.