should i use a 2.5 gallon, or a 5 gallon or a 10 gallon sump/refugium for a 10 gallon tank?

To use a sump the tank must be either drilled or have an overflow unit, you don't have that, do you?

But, it is possible to use a large power filter as a refugium if you put some live rock chips in it and treat the life in the filter carefully and avoid killing or drying it out when you clean the pump motor every month or two,

I still suggest you start the tank with some live rock and become more adept at doing water changes, matching salinity, temp, maintaining alkalinity and calcium. Once you are better at that, then think about your next tank which might be set up for more advanced methods like sumps, refugiums, larger fish and corals.