
The setting for Filter Results by Size is located at Settings->Sorting & Filtering.

When you enter your internet speed (in Mbit/s), Red Light will determine the maximum allowable size of results based on the duration of the Movie/Episode.

e.g. If you set your internet speed to 20Mbit/s, then the following maximum filesizes would be used by Red Light:
10min = ~1.35 Gig
30mins = ~4 Gig
60 mins = ~8 Gig
90min = ~12.5 Gig
120min = ~16.2 Gig etc...
This is done on the fly, so any duration will have an equivalent maximum file size.

Try it out to see...