that's a vague and open question... but really depends on the type fishin you plan to do and how it will work/fit your needs. Not knowing this or the boat your after... it's hard to say...
One thing though, is... You always want to go as big a boat with close to max hp as the budget allows... Ya never hear anyone complainin about too much horsepower... or too much boat..
Usually 21's come in the 150 to 200 hp ranges... so a 21 ft boat (not knowing what brand/make/model) should pretty much scoot out really well... but speed isnt everything... You'll need to consider deadrise, draft, wet weight and all them other variables... Not knowing what 225 motor.. I would say that's alot of motor and the boat better have enough fuel capacity to feed it..
Without specifics... it's hard to say other than being "general" about suggestions.
Good luck on your boat purchase!