I'd say either the 5 1/2 x 5 1/2 or the gallons for the various vine crops. I've done these many times in gallons so as to set out pretty large plants, and it works well.

I'd really caution about the sunflowers, though -- if they get stunted in the flat/pack/pot, that's usually it, they just stay small forever. So, either really big pots, OR don't start them as quickly -- I'd say no more than 3 to 4 weeks before transplant if you do smaller pots. BIG pots (1 or 2 gallons) would probably be OK for longer growing period, as long as you don't overcrowd.

Thanks denninmi it helps a lot not having to guess my way through. I think I will go with the 5 gallon pots for sunflowers. I don't only want to get an early start to start early. I ordered seeds from five different companies. I already started tomatoes and tomatillos. Some of the seed packs from one of the sellers were not viable. If this is the case with the other seeds I ordered from them I want to know in plenty of time to order replacements.

Why are you starting so early? Where are you located?

stuffradio,

I'm also in zone 9 and with heated greenhouses we start early because the last frost dates are usually end of Feb to transplant outdoors. The heat of summer brings everything to decline. We do not get the overnight lows in summer needed by plants to set fruit.
EXAMPLE; tomatoes need overnight temps below 73F to set fruit. Summer also brings on the myriads of bugs that will chew the plants right down to the roots.

Lou

stuffradio I live in Arizona in zone 9B. Feb 15th is the last threat of frost. Some of my low chill fruit trees usually break bud before that date. Our county extension recommends Feb 15th as the time to start putting out transplants and sowing seed. They also recommend starting tomatoes and such 8-12 weeks before transplant so for some of my crops I am not early, I'm little late.

Oh I see, I'm in zone 8a or 8b and my last frost date is mid Aprilish. I rarely get snow/frost then... but I have had a few cm of surprise snow on Easter a couple times in the last few years. March is the time to start most of the stuff here but Brussels Sprouts and Pumpkins etc. are started April/May.