Probably as tall a pot as you can manage with a wide open top , in terracotta or a glaze in a colour you like.

Every garden centre has pelargoniums ( geraniums ) in every shade of red , pink or orange and white too. They are good for the centre and then some trailing lobelia and bacopa round the edge.

That's keeping it really simple but hopefully easy and reliable too.

Any general purpose compost is fine.

If I may add to Inverglen's comment, generally speaking, whether planted with annuals or perennials, pots need feeding on a fairly regular basis throughout the growing season to keep them healthy and flourishing. You might like to add water-retaining gel if the pot is not a very big one, otherwise you may end up having to water a couple of times a day, though that is possibly more relevant to hanging baskets. If the plants in the pots are perennials and will be staying there year-round, make sure that in winter the pot is taken into a greenhouse, or is put into a sheltered spot and lagged with something like hessian, because the roots are susceptible to freezing when not buried in the ground. In summer, really warm weather can also mean that the roots get hotter than is good for them. Take care that they don't cook. I shall leave it to the really clever people to write you a list of the hundreds of plants which will grow happily in pots, or you may like to have a look at some of the members' photographs for ideas and inspiration. Welcome to GoY!

Don't forget to ensure there is plenty of drainage at the bottom and a drainage hole. It's also best to have the pot on feet so it is able to drain excess water away.

I think the pots should have shade at some part of the day. Put them close together. Otherwise you will be incessantly having to water them. One way to keep plants in pots nourished, is to have a weak solution of miraclgro in some type of fine sprayer. If you have the time .....mist them with this spray whenever you have the time and inclination. I can tell you from my own use of this method....they like it. They will reward you with lots of flowers. Most composts are worth a try. I have an enormous English Lavender. Roses/perlargoniums/ fuchsias/Heucherers do well in larger pots....specially Marmalade and Sweet Tea. Even Bonariensis likes a decent pot/ Crinum/Agapanthus. Avoid curved pots to make decanting easier. Most of mine are pottery and plastic.

I've got pottery and plastic ones, too. The first ones can get awfully brittle in a cold winter, and the latter, unless quite expensive, don't really look as nice. I got some all-weather terracotta-coloured paint a few weeks ago, and have gradually been painting all my plastic pots so that they match - no-one would suspect how cheap they were in the first place. Sounds awful and tacky, but works remarkably well. I've been planting big pots full of Marguerites to place around the garden for the summer, and they look great. Dahlias, begonias, pelargoniums and chrysanthemums work well, too, as long as they get to overwinter in a frost-free, dry place. If the pots are big enough, you can even grow small trees in them - fruit trees, bay, rose trees, etc..

In order to answer your question, more info is necessary. I get the impression you want more permanent planting in pots, in other words, not just summer bedding displays?
How much room for pots do you have, and how many pots would you like with permanent planting in them? If you wanted, say, 6 large plants in pots, that's a completely different thing from smaller planting in larger plots, so a clue about what effect you want to achieve would be good. Do you intend to stand all these pots in the same place, clustered together, or dotted about? The conditions (sun, shade, wind, cold) make a difference, and as for potting composts, different plants need different composts - Camellia, for instance, will require ericaceous compost, whereas Fatsia will not.