Hi BLOD,
You're right, drainback's not an option with evacuated tubes, so pressurised only. You can either use a high temp glycol such has Tyfocor LS. This is better at handling higher stagnation temperatures and won't sludge up or degrade as easily.
Also, activate the recooling/holiday option on your controller during Spring/Summer months to minimise stagnation.
Also, when filling the system, consider pressurising a bit higher as this will raise the boiling point. Remember to adjust the expansion vessel precharge accordingly and note the 6 bar rating on your PRV! All this depends on the manufacturers recommendations of course.
Back to the radiator size, you could stick the biggest double in you can fit (in loft maybe?) and divert with something like a Resol VA300 valve.
Cheers,
Matt