Unless you are careful, it is easy to create models without any notion of their scale and consequently end up with a set of objects that are disproportionate in size when they are imported into Unity. Check the units your 3D package is using and adjust the export settings so that the size of the model is in correct proportion to the cube. Do a test import and compare the size of your imported model with a “meter cube” (the standard Unity cube primitive has a side length of one unit, so it can be taken as a 1m cube for most purposes). A lot can be learned by studying existing 3D character meshes to see how the topology is arranged and why. A poor topology will not allow the model to move without unsightly distortion of the mesh.

The exact nature of a “sensible” structure for your mesh is rather subtle but generally, you should bear in mind how the vertices and triangles of the model will be distorted as it is animated.

Although this is a whole subject in its own right, there are a few guidelines you can follow to ensure a model works well with animation in a Unity project. This is the process of creating your own humanoid mesh in a 3D modelling package - 3DSMax, Maya, Blender, etc. There are three main steps in creating an animated humanoid character from scratch: modelling, rigging and skinning.