Zinnias seem to grow anywhere, even cramped places
The first year I grew zinnias was in 2011. I decided to try growing them from seeds, when just on a whim, I decided to buy a $1.79 pack of Burpee zinnias. I bought the Giant double flowered mix seed pack and a pack of Purple Prince zinnias. Both were advertised to grow between 26-30" with the giant double flowered mix, and 36" for the Purple Prince type. But from my experience growing them in 2 different properties is that they grow to 5-6 feet tall, even with medium sun at one spot and full sun where they are now.
Unfortunately, I didnt really understand Burpee's rudimentary planting instructions on the seed pack the first year, so my interpretation was that I was supposed to plant 1 seed every 1/2", and plant them 1/4" deep, then thin them out when they get larger. So thats what I did. I managed to plant 1 row of seeds about 30" long before I ran out of seeds.
Then they germinated in about 3-4 days, and started growing. They quickly became crowded, so I began carefully digging them up so that eventually only 1 plant was growing for every 12" of row. I lengthened the row to about 5 feet long, and started a 2nd row beside that one. But as I dug up the excess seedlings, it meant I had a lot of them left over, and I dont like killing perfectly good plants by tossing them, so I, ummm, temporarily transplanted them into places where I could later replant them once I, ummm, found space for them.
Problem was, I coulnt find enough space for all of them so I ended up leaving some of the extras planted in pots alongside larger plants that had been growing in those pots for years, such as my seed-grown pomegranate bush. Truth is, these pots really didnt have extra root space, because the plants in them had taken up most of the space with their own roots. But that didnt seem to matter to the zinnias, as the ones in the pots grew just as large as the zinnias in the ground.
Unlike many plants that are at least partially dwarfed when grown in pots or in any tight space, zinnias dont seem to be affected by a lack of root space for the most part. I have a zinnia that popped up in between the concrete pad of the front porch and the 4" deep edging, and there is only about 1/4 to 1/2" of space between them, but a zinnia is growing out of there, and it is growing just like the ones in the ground in front of it!
They are also very easy to grow from seed, as they germinate quickly, and they seem to be rather beginner friendly, except for the staking you'll be forced to do if you grow the 5 to 6 foot tall types, which produce lots of lateral branches that get heavy with flowers and start to sag and fall over.
I grow a lot of plants as the owner of a small landscaping design company, and my primary flower growing interest is dahlias, but zinnias are simpler and are cool for different reasons.