Some of the most commonly cultivated Eucalyptus species as garden plants in cold temperate climates are fundamentally chosen because of their reasonable tolerance to winter frosts. Many of these species, including Eucalyptus gunnii, can become medium to big sized trees if planted out and left to grow "at their will". However this is not always the desired outcome for tiny gardens, and surely not for pot grown plants in urban "yardless" gardens. Is there something that can be done? Yes.
If the eucalypt species you decide to grow is a good coppicer (not all of them are), you can try to train it from the beginning or convert a plant getting too big sized into a small multistemmed shrub looking like specimen by severely cutting back to almost soil level in the right time of the year. With proper care afterwards the new coppice growth will soon fill up. But perfect symmetrical and compact growth is not always achieved by letting it grow again "at their will". Some training is needed, and this involves trying to get as many branch divisions as possible. One way to do so is clipping the terminal growth in some branches of your choice, trying to make sure you remove the very top part only and you do that when new lateral branches are already emerging and well visible a bit down each stem. This operation can be repeated along the same growth season two or three (or more) times depending on how fast the growth rate of your plant is (which depends on how much you take care of it, how much light and watering it receives, and fertiliser dosage).
Here you have an example of a coppiced E. gunnii in a 15 litre pot getting ready for formation pruning. Target is either keeping it as a potted plant for years trying to achieve a compact outlook or planting it out as a low shrub and keeping it like that at the garden. Not a miniature bonsai but some of its principles also apply here.
Depending on where and when you start training your eucalypt tree the final visual outcome can be different. You can also obtain medium height globose sculpted trees!
Please remember there are some cautionary measures to take before using secateurs!