The main issue you'll face is that very few conifers will stay that small.
Even so-called "dwarf" conifers are only considered such when compared to their
larger relatives. For example, a dwarf redwood might still grow to be 200 feet tall.

What you will need are mini conifers, which will still need to be tended periodically.
Mini conifers, in general, fetch a higher price and are usually grafted. Also, they are
very slow growing (in order to stay within the size requirement), so you may not have
that instant gratification. In other words, you can't plop a full-sized Christmas tree
into a pot and expect it to stay that way. If you don't mind changing the tree out,
and replacing it with something in a few years, then your choices become much broader.

Josh

I think too, that your USDA zone is going to eliminate the lion's share of good possibilities. What zone are you? It's good to include that tidbit in your user info so we can better help you.

Al

dwarf blue spruce, though they will get to about 7 feet eventually, and will probably get root bound in a container. Also they will sometimes revert back to their original genetics, creating a very odd looking tree.