Your car already has the injector which most smoothly delivers the amount of fuel that the ecu requires for the given amount of air available and the throttle input it is receiving will determine that rich/lean level, and also airflow into the throttle.
Increasing the available fuel assumes you will also be increasing air flow, ie; larger turbo and pipes.
Also, to benefit from larger injectors, you will want a larger pump. The small Subaru pump cannot keep up with the demands of a reasonably upgraded fueling system.
What other modifications are you doing along with this?
There are several levels of injectors, and several levels of pumps, but all of what you choose should be planned together...
for example... you want ^Z amount of power...
that requires ^X amount of air and ^Y amount of fuel
balancing for ^X means a very efficient turbine, and pipes to move air for it the best way possible.
^this is not cheap, and takes some knowhow, which really isn't entirely rocket science, but it mostly is...
this basically means you can get away with 180hp without a lot of finicky or expensive stuff ...maybe even 200hp... but I'd be looking at the whole system together, and all the plans should be laid out at once. It will take $500 to get over the 250hp point, $750 can net you 275hp with the right shopping finesse, like a bargain intercooler (not an ebay crap one, i mean a used process west or grimmspeed unit) and maybe even a used turbo if it's not beat w lots of shaft play.... those will make wanting pump and injectors functional... i hope this makes sense.
If you have a list of planned mods, or parts already ready to install, injectors can be had from injector dynamics.
they make a reliable replacement and a very fine upgrade.
you can run as large an injector as you like, the ecu will still feed the same amount of fuel and compensate for the A/F the same as it did, you will simply have massive headroom if you buy giant ones, and maybe have some linearity issues.
It's good to stay within a certain range for each power level, i think there's a chart around somewhere as to what injector is best for what power level we ask of our engines, most techs have their own kind of 'rules of thumb for injector sizes' which i do, but i refuse to let my opinion blow your engine.
tell us what your total end goal is, and im sure we can be able to tell you a good pick that will match the setup you want.