In my personal experience, anything but the most modest of roses can be kept for long in an 18" diameter pot. I have Miniatures that are too large for such a pot, if the intent is to house them for years in this manner.

However, here are a few very well-scented small roses I feel will prosper for at least 5 years in a small (18") pot:

'Sweet Chariot' (definitely at the top of my list)

'Basye's Blueberry' (may reach 3 feet)

'Château de Clos Vougeot'

'Gloire de Rosomanes'

'Marie Pavie' (will eventually outgrow a small pot)

'International Herald Tribune' (great for pots, medium strong scent)

'Pretty Jessica'

'Souvenir de la Malmaison'

I'm sure there are others that qualify, but this is what comes to mind.

I dare say that even 'Fragrant Cloud' could be grown own-root in a pot that size for at least a few years (avoid grafted, because it will grow too large)