You can use a number of methods to resize things in ZBrush, all of them can be tricky.
Assuming you have these two (or more) objects as subtools you can do any of these.
The first method is the newest and seems to be the most stable and straightforward:
In the Geometry Palate look for Size, XYZ should do the trick. You very likely will need to use Position sliders just above size as well.
Next is the Deformation Palate, again look for Size slider and set it to XYZ (little tiny letters which should already be on). However, dont trust the deformation palate, it causes crashes some times depending on what you do with it.
Again, with this you will likely need to reposition stuff, Im guessing the boots, in this case using the Offset sliders, this is not easy and can get frustrating.
Last is sort of the hardest to learn, but best to know in the long run in my opinion, this is the transpose tools.
They are at the top next to Edit and are Move Scale and Rotate.
I, after years of using them, still sometimes find them very very cumbersome, but when I get it right they work great.
You will need to use both Move and Scale and take your time.
When you draw your transpose line it will have three little circle thingies, you want the outer one most likely, the other two make life very complex, and to be honest, I literally never use them.
If you watch a few of the videos on the Transpose tools it can help, or make things worse, I learned them by messing around and yelling a lot.
These are seriously good tools to know, but work in a rather unique way, I used to hate them but now think they are wonderful most of the time.
Actually while I was typing this I thought of a third method, one which I think I shouldn’t mention, but I will anyway, but leave it to you to look up:
This method is sort of cool, it would allow you to put the boots anywhere you want any time you want.
Learn how to make an Insert Brush, turn your boots into one and then, if you make a new figure, say a centipede, you can quickly put hundreds of boots onto it (in theory!)
Insert brushes are a topic of their own, and I would take your time being new to ZBrush before diving into that can of worms.
Cheers!
Mealea