First, not all things can or should be transferred into the same size. That normally isn't a problem because not all of the things you list are started at the same time since they have very different plant-out times.

Second, the size you pot up to is partially determined by how long before you can plant out, how long they need to be held waiting. Don't know your location and that info would help sort this out. But on average broccoli, cauliflower, and celery don't require potting up as they are normally started just a couple of weeks prior to planting out. Peppers and tomatoes have much longer inside periods as they are planted out much later. Many transplant them from the germination tray into 3" pots first so you could use the 36s for them or even the 18s.

Need more info from you to be more specific.

Dave

Thanks! I'm zone 5b NW Indiana. The tango celery packet says 10-12 weeks indoors, I started 2/9. I was planning to give the broccoli and cauliflower 5 weeks, though the packet says 6-8. Seeded those 2/23. Started peppers 3/3, and I haven't started the tomatoes yet. Do these time frames seem good to you? Also, I was planning to start my broccoli and cauliflower in raised beds covered with medium weight row covers over hoops the last week of March. Last frost free date May 2, will the fabric keep them warm enough? If I keep the broccoli and cauliflower In the 72s for the whole time, would the seed starting mix give them enough nutrients? Thanks for all your help, I've been using quite a bit of your advice I've found on the forums for quite a while now. :)

Row covers raise the temperature by 2 to 3 degrees. Although broccoli and cauliflower do not freeze easily I think pushing it by a month is not good. I think I'd wait a couple of weeks. The temperature of the soil is most important when transplanting or planting seeds outside. If the soil is still cold seeds or plants won't grow and may rot. Use that 2 weeks to warm the soil. Put plastic over your raised bed or put your row cover on 2 weeks early. Both will help to warm the soil

<I was planning to give the broccoli and cauliflower 5 weeks, though the packet says 6-8.>

Agree with what dowlinggram said above. In most cases those guidelines on seed packs are for direct seeding in the garden rather than for growing transplants indoors. In zone 5b your plant out date is mid to late May (plant out dates are an average of 2 weeks past last frost date) so while you may be able to get away with planting the broccoli and cauliflower late Apr to early may (with protection as needed) the tomatoes wouldn't normally go out until the 2nd or 3rd week of May depending on the weather and the peppers would go out approx. 2 weeks later.

So you have a lot of things started way early. Broccoli, celery, and cauliflower, if you can keep them going and healthy that long, will likely need more than one transplant over that time so just work your way up the sizes of cell packs you have. Transplant the peppers into the 18s and since they are slow growing hopefully they will do ok in them until plant out time. Tomatoes, started at the proper time, normally go from 72s to 36s to or into solo cups or larger.

All this depends on several variables - lights, leggy plants, nutrients, ambient air temps maintained, etc. etc.

Hope this helps But next year plan to start much later in your zone.

Dave