Okay, having watched Thirdspace last night...
Only the small Thirdspace fighters ever actually come through the portal. They look like modified Vorlon fighters, with tentacley bits at the front but metal "fins" on the back. They seem to be a bit larger than a Nial. One of them is roughly equivalent in power to a White Star, in fact one of these and a WS go 1 on 1 and both blow each other up, so I'd say they are pretty much evenly matched. That puts a Thirdspace fighter at Raid level, one fighter per flight (like Sky Serpent or WS Fighter) and 1 flight per wing.
The cruiser that tries unsuccessfully to come through, is much much bigger. I'd say at least 10x as wide, 10x as deep and 10x as long as the fighter. That puts its volume at 1,000x. Assuming its power is proportional to size, that makes it 1,000 times as powerful as a fighter... which puts it 10 Priority Levels above Raid.
I think we'd need a few more PL's to introduce the Thirdspace aliens...