A traditionalist would argue that one should always practise outside, preferably in a garden or park and whenever possible in the early morning. I have known practitioners over the years who would refuse to practise indoors and instead risk frost-bite or getting wet in the rain. Good for them; but that's not for me. Where I live it seems to rain or snow a depressing amount of time, year-round, and the ground outside is often too slippery to risk doing solo forms. At my age, a broken hip or wrist is no laughing matter -- no matter how good your sense of humour.
So, as my stealing a phrase from Star Trek would suggest, the answer for those of us who are geographically-challenged by the weather is to look for enough space [indoors] to practise safely and in some comfort. However, you may already know that isn't easy if you live in a small apartment or the wife objects to you swinging a sword in the living room while she is trying to watch tv. Of course, some apartment buildings have recreational rooms or gymnasiums that can be of some use for regular practise but your fellow tenants may object, overtly or otherwise, to your taking up space with your slow-motion/strange movements, especially if you are using a staff, broadsword or sword.
I know from experience in teaching many introductory taiji courses in fitness and community centers twenty-five odd years ago [the photo shown was taken at one such in the early 1990s] that one of the most common complaints I heard was that beginners were interested in practising but had no space to do the solo forms at home and were too shy to practise outdoors, even if the weather permitted. Perhaps that is one of the reasons that qigong and yoga have eclipsed taiji in modern popularity is that it is easier to learn very short sequences of movements and postures and much easier to find indoor space in which to practise.
While it is always preferable [especially in one's early years of training] to practise a competent long form, sadly they are often prohibitive of the amount of floor space necessary indoors. You can sometimes train these in a confined area by stopping, moving back or to the side a few paces and continuing from where you had stopped; but this can be confusing and also interrupts the 'flow" of movement that is such an intrinsic part of the taiji experience. More experienced practitioners can also use what are called "Changing steps" which allows one to move forward in principle but while doing so on the spot. One famous Yang style master was famous partly because of his party trick in which he would do an entire long form on a table top simply by constantly adjusting his stride in this way.
In some ways, the most practical solution is to learn a competent short form if you have to practise indoors most of the time and have some free space to use [coffee tables and chairs can be moved and stacked along a wall temporarily if you train yourself to ignore the irritated glare from your spouse or the cat that wanted to have a nap on her favourite chair].
However, you re-arrange your environment to make room for regular indoor practise; remember the words of a Yang style expert, Sam Masich, who said to a group of workshop attendees in 1990. His words that "You can correct anything in your practise eventually except for a lack of regular practise." was as valuable advice then as it is now. By the way, he's still teaching today and has achieved some deserved fame in the taiji world. I'd recommend his classes or workshops; and, no, I don't get rewarded for this endorsement.
Copyright Michael A. Babin©2018
43+ Years of Experience Training in the Chinese martial arts; 33+ years experience teaching taijiquan and 24+ years experience teaching baguazhang