Over time they will do anyway - they're evergreen perennials rather than shrubs, and as such, increase at root level in the soil, meaning they need digging out and splitting periodically, especially in a pot.

Finding smaller ones isn't easy frankly - the basic difference between the large and small is, Phormium tenax varieties get very tall, Phormium cookianum varieties are much shorter. The trouble is, when they're on sale, they don't use the full name when giving the varietal name, so with some its hard to know what you're buying. Reliably small is Phormium 'Cream Delight' or 'Yellow Wave' and a red one called 'Sundowner', as well as 'Black Adder'. The best thing to do is look at lists of Phormium for sale on line, checking ultimate mature heights if they don't mention tenax or cookianum, then seek out the ones you're interested in.

Bearing all this in mind...have a look at Dianella!