I have Lanceleaf Coreopsis (Coreopsis lanceolata) and the bloom is about 2 inch diameter, and tickseed coreopsis (Coreopsis tinctoria) which is more like an inch across. I love the look of the Lanceleaf, but tickseed is much more drought tolerant.

How tall will the tickseeds get

I really think it depends on the soil and water you give it. My tickseed, aka Goldenwave, only gets about 8 inches tall. But I have it in a rocky area that has just a thin layer of soil and it gets almost no extra water. My book says they can reach 4 feet. My lanceleaf gets about 2 feet tall and I water it once a week. I gave some seeds to a neighbor to use at his ranch and he said they barely grew at all. But his soil there was sandy and he didn't water it.

Local nursery suggested morning tickseed for the height I am still looking for. Not reasy yet. But wondering if there is another tall yellow plant instead, or variety. Need 3-4 ft tall against my fence.full sun, I mean full.

A quart or gallon sized Esperanza/Yellow Bells (Tecoma Stans) will take full sun and can reach 4 or 5 feet in a season. Unfortunately the roots are not reliably winter hardy in clay north of 8b (or 9?), but the long robust blooms could be worth a try in your location. It is sold as an annual in Dallas. Narrow leaf esperanza (T. stans var. Angustata) is a bit hardier if you can find it.

Oh... forgot about the yellow varieties of the native Mexican hat (ratibida columnifera). Starts blooming here in late April and repeats through first frost. They grow a bit over 3 ft, though we usually cut them back a couple times during the year to keep them looking fresh. They keep a nice large green rosette throughout the winter. So easy to start from seed that volunteers can become a problem, especially in unmulched/unmowed spaces. But otherwise a tough, trouble-free, and attractive plant.