Welcome to BL
There is actually a big
thread on growing that might provide some useful general information. In regards to your specific questions:
Q1 Carbon filter? I'm wondering what size I need, I'm guessing a 4" for a small area and only 2 plants. And is there a better make than others?
Surface area is what matters with carbon filtration, and the stuff doesn't work forever so you need to replace the filtering material every few months.
Phat filters are known for being a good mass-market solution for grow room odour control, on their site you can find specs for each type of filter that measures scrubbing potential in terms of CFMs. You can DIY your own filter with ducting parts to look and function like a Phat filter but you need to fill it specifically with
activated carbon because carbon matting like you can get for air filters is geared more towards trapping particulates and won't provide adequate odour control.
Getting back to CFMs... you really have to carefully spec out your fan(s) to ensure they are capable of a good enough combination of
static pressure and
cfm (total airflow) to create a small vacuum in the grow area so that all the air inside goes ONLY through the filter. Since a carbon filter adds restriction, having a decent amount of static pressure is key so best to shoot for overkill with a fan that reacts well to overvolting and then maybe use a potentiometer or some other means of controlling the fan speed to dial in the optimal level where pressure is good but noise is also kept as low as possible.
Q2 Low Odor seeds? Stupid question, but how low odor is low odor? Compared to normal seeds is the quality of the grow as good?
Best to just take the time to design the scrubber and exhaust setup properly so that any and all odour is scrubbed completely and not just "managed". It doesn't matter how stinky a strain is, all that really matters is whether or not you can smell it and that all rests on the functionality of the scrubbing setup.