I don't do solos, I use .75g pots for seeding then transplant into my 5g pots at week 3ish. The .75's are a good size, when I fill with my Pro-Mix, I've pre-wetted the medium and added some Mykos and that will stay damp enough for the seeds to sprout and go for about a week before needing a bit more water. I water close to the sprout and out about 75% of the way on the first watering, then the next time do the entire pot. The pots are taller which gives the roots a chance to go deep right off and at 3 weeks they've made it out to the edges of the pot.
I honestly wouldn't worry about it at 5"...I previously transplanted solo to 5g bags. Get your soil damp/wet before the transplant so you're transplanting into soil that's ready for roots to explore and is loose and damp. By the next time you water, your plants will have established pretty well.