My dining halls have become legendary with very little effort on my part. I tend to make them fairly large... I guess. They seem pretty small when I have 150 Dwarves and about a third of them are cramming themselves into the meeting hall between tasks, but they seem enormous in my first year, when I'm digging out vast amounts of space that won't be filled until a few years later.
... Anyway.
I just dig out a big room, put X chairs and X tables in it, and sometimes I don't even HAVE to engrave it. At least, I don't remember engraving my last one, and my Dwarves still end up with "dined in a legendary dining room lately."
I wouldn't worry about trying too hard. Unless you're making a lot of small dining rooms, or the ones for your nobles. Then it's a matter of masterwork. Masterwork chairs, tables, racks, coffers, and cabinets can do a lot for a room, particularly if you let somebody masterfully improve them with spikes of shell and masterfully encrust them with gems. ... Not that I ever bother trying to MAKE that happen. Ugh.
But uh... more to point: A 21 x 11 room? You'll have no problems whatsoever. You may not even have to put much furniture in there. Smooth it, engrave it (even with novice engravers), and then start tossing chairs and tables in. It'll be legendary in no time. If you really want to max out the room's value then you have a lot of work ahead of you, but a legendary dining hall is exceedingly simple, especially when it's that large.
Happy Dwarfing.