If you have been working on a 13in screen for awhile then the 21.5 in iMac will probably be a welcome upgrade for you, but if it is financially possible, you will not regret getting the 27 in. The screen is unbelievably nice and is great for graphic design. Of course this will all come down to a budget, but here are a few recommendations. I personally do graphic design and video editing and I have the 2017 5k 27in. with a 4.2 i7, 256 ssd, 4gb graphics and 24 gb of ram. If I had it to do over again, I would have gotten the 512 ssd, but otherwise I am happy with my machine. See below for more detailed info on each thing.
As far as specs, it will depend slightly on what programs you will be using. Photoshop and the like will run fine on an i5. Video rendering will be improved with an i7, but you will get some serious fan noise during export (it doesn't bother me, but some people can't stand the tiniest bit of noise for their workflow). As far as clock speed, higher is better and you won't regret having more speed if you can afford it, but I personally wouldn't go for the high end one unless you are planning to do larger scale videos.
For other specs, Ram is always nice. The 21.5 in that you were talking about does not have user upgradable ram so you will need to buy it originally. You could go crazy adding ram, but I expect 16 gb would be find. If you go with the 27 in, you can upgrade the ram yourself so get the 8gb model and you can either get another 8gb or 16gb of ram and add it very easily.
The GPU is good to have especially for video work, but it's not going to kill you one way or the other. Depending on what you do with the other specs, you will probably be at the 4gb card anyway which should be fine.
Also, make sure that whatever you do and if you only do one upgrade then make sure it is getting an SSD. A few machines can still get the hybrid thing, but you absolutely will want an ssd. You will almost certainly need external drives no matter what, so I would say get either the 256 or the 512. 256 is ok if you don't mind occasionally cleaning things out and working mostly off the externals. The 512 will give you a little more room if the money allows.