You can actually use any speaker whose impedance is equal or higher than the minimum the amp can support. How much wattage the speaker needs to handle depends on how careful you can be, how loud you want to play, and how big the room is. You can use 10 watt speakers on a 250w amp but the possibility of blowing the speakers is there bigtime. You can always use speakers that are rated for more power safely but won't be able to play them as loud as they can play.
To get a speaker to play 3db louder requires twice the power so often it''s not worth maxing out the volume for the additional cost of a bigger amp.