Oxalis purpurea, probably 'Grand Duchess White'. One of the least aggressive of the oxalis. A good ornamental. Joe

You said no larger than 5x5, but you didn't say a minimum size. There are several varieties of gardenia that stay under 5 feet. they are also evergreen and very fragrant. G. 'Mystery' is said to seldom get over 4' but it does say it can get to 6-8' on occasion, but I would think that would be easily controlled with pruning. I personally have not grown that variety. I have August Beauties which I keep pruned to about 5 feet, but they are only 3 years old, not sure if I am going to have trouble keeping them that size eventually. There are lots of other varieties of various sizes. You also don't say where you are located so not sure if those are suited to your zone. Here is a link that might be useful: mystery gardenia

It may or may not be Ipomoea batatas (sweet potato) but definitely looks like an Ipomoea of some type. (The genus also of morning glories, moonflower vines, and a myriad of others.) Their leaves can wildly change shape and size, among individuals of same type, and in phases on a same plant. I would lean toward I. batatas since it's not blooming now. Most Ipomoeas are in bloom at this time. I. batatas can bloom, but often chooses not to. The amount of sun it's getting may be affecting the status of blooms, if it's one that would otherwise be blooming now.

Not sure if this is the right forum to post this ==>>> you may post anywhere you want ... but as you note.. you already know its a sunflower ... when it flowers... and you want a more specific ID... then it would be here ... but if you want to find peeps who grow annuals ... and talk about sunflowers ... that might be the annual forum ... though that forum is rather dead this time of year .... but do understand... its winter.. for a vast percentage of us on GW ... and a lot of the forums become rather inactive ... so post where ever you can find an audience .. have fun in the garden ken