It is a bit oxymoronic. Big octa's that I have looked at folded inwards, but their rods did not fold down, so they were perhaps 200mm around, but over 1000mm long.

Then there is the issue of holding it up, so there is a sturdy stand, and the need to support the octa speed ring off the stand, and hang the light head off the speed ring.

I do portable location work for portraits. My light modifiers, backdrop, stool, light stands, black window blocking sheets, sundry a clips, gaff tape, etc all go into a 400mm x500mm x 1200mm foam padded nylon flight bag. My heads, accumulators and batteries go in a second (heavy) suitcase style case.

I ould not worry about the size of your modifiers until the rest of the supporting kit;'s needs are worked out.

That said, yes, they are inter-related, so some iterations are needed before modifiers, light sources, and stands all suit one another and the format in which you are working.