The size you should check befor purchasing

1, Dropout.

The dropout also means the places of a set of bike frame to catch and fix the bicycle wheel's axel, and there's two type: Front Dropout, and the Rear Dropout.

Actually I think it's for fits with the wheel hub, so both dropout should measure the inner side to the other inner side.

1.1, The Front Dropout should measure the axel hook's inner to inner, and you'll get a width between them. And for Voilamart normally front wheel conversion kits they would fit with 100mm Front Dropout (about 100~103mm or 3.94~4.00in width).

1.2, The Rear Dropout also a width between both inner sides, and for Voilamart rear wheel conversion kits they would usually fits with 135mm Rear Dropout (about 135~137mm or 5.315~5.39in width)

1.3, Also theres some wider dropout type for Fat Tire Bikes, Beach Cruisers, or Snow Bike. These would be different in the parameters, but you still can get them by a ruler, it's easy.

2, Wheel Diameter.

From the bike frame, you can get the wheel diameter by measuring the length between axel dropout hook and the bottom bracket(for rear wheel) or fork bottom(for front wheel), just like the picture above shows. In this way you'll get a "Half Diameter of Wheel", so you should know the diameter by "2 x Half Diameter".

But if you've got a whole bike, maybe it's easier. You can just find the tire info on the tire, they've been mark on the tire surface and shows as form like "wheel diameter X tire width".

3, Tire width.

Yes, the 3rd I'll mention is the tire width. It's measured in much the same way as the dropout, but the measure way will be closer to the bottom bracket and the fork bottom. You should figure out the Max Awailable Tire Width of your bike frame, and then choose the right tire.

If you install a over-wide tire on the wheel, you'll mostly find that the tire will get touch with the bike frame, and the wheel would been fixed, unspinable.