First, don't drill anything you haven't been taught in person by a good teacher. You can learn a lot watching videos, but videos don't give you feedback and correction. I'm worried that you'll be drilling the wrong movement, and that will make it hard to undo later on. Drilling should only be done when you're getting feedback and are actively making improvements to your form.
Without that feedback, without a teacher actively giving you correction, you will not be able to refine your movement (at least not very well). With kenjutsu / iaido in particular, it's all about refinement of physical movement and working on very fine details and subtle, hard to see qualities. You're not going to help yourself by drilling anything new on your own.
You can still drill what you've already been taught by your teacher, so as to not lose it. For that, you should consider practicing somewhere else, somewhere with a higher ceiling, or outdoors. Yes, you can use a smaller sword if you need to. It doesn't change much having a smaller sword. Mostly you're just using it to visualize the technique. Consider the change you're making using a bokken instead of a real sword, for example. It's just for visualization. The actual feel of the sword, its weight and center of balance, will change when you use a real sword.
Consider also using this time to work on things that don't actively involve sword practice but which support it. Work on your strength and your ability to calm your mind. These complementary exercises are a completely different subject that I won't get into here. But if it were me, I'd be working on these exercises 75% of my time while I don't have a teacher actively teaching me. The other 25% of my time would be spent on retaining what I already know, but not drilling it too hard (because drilling bad habits and mistakes will not do me any good in the long run).
Hope that helps.
