getting a new grow room

im planning on a SoG method
using 2 600w hps, and grow room size will be 10w x 5d x 8t...
it will be on 4x8 grow e/f tray, with 2x30g res


i plan on using the vent in the room to redirect airflow into the garden,
and another fan to pull air through a scrubber--->lights---->fan---> outside

is this a good idea? if so, what size fans would you recommend?

sounds like a good starting point. there is a great thread in the growroom design & equipment called ventilation 101. great read and will have all the info you will need to figure out venting. remember to run your setup for a week after its all built to make sure you have everything dialed in. remember to record all your data so you can spot trends or problems before they become major. some things to keep an eye on:
temp at the intake and outtake. temps in the growing area during lights on and off. temps of water/rez. RH% on intake, outtake, and in the growing area. rez pH over the week to get an idea if it will remain stable. Burn a strong smelling candle inside to see if your scrubber is ideal. etc etc etc. doing this now will help insure a successful grow with very little problems

I would put the scrubber at the end of the run. Any leaks in your ducting will get sucked in since you are pulling air. Move the scrubber to the end, or put the fan right after the scrubber pulling through it, then pushing through the lights

I think a 6" fan would be awesome. You should get away with smaller, but the cooler you keep those lights, the better. Plus your hoods will be 6" anyway, and if you go with a smaller fan, then you need to get adapters and you haven't saved that much.

Why 2 30 gallon reses? Why not just a larger single res?

If you plan on using a scrubber, IMO you will want to test it in your system and make sure it will work for you. If you dont want to use it till mid-late flowering thats your choice, but do you really want to wait till you NEED it before you see if it will work?