A full tile in TT is 32 pixels long in X and Y directions. But that's all I'll explain about tile sizes . Now I will explain the exact sizes of train sprites. Below is an image which shows the sizes of a normal passenger coach in TT. The image is in 1:1 scale.

The / and \ views are 8x2 pixels long (every pixel the line goes up, it goes 2 pixels sidewards, so 8x2 pixels is actually 16 pixels for the whole line). The width of the / and \ views is 3x2 pixels. The | view has a front of 7 pixels high (this can be a pixel higher for doubledecker trains). But if the train has a sloped front, the front in | view takes more pixels (the length of the roof will decrease then). The roof in | is usually 16 pixels long. _ view is 32 pixels long (this may be 31 if this fits better). The side in | view is 7 pixels in height and the roof takes 4 pixels.

A train sprite doesn't have to be this size. The sizes mentioned are the maximum sizes. With TTDPatch it's possible too make shorter train sprites up to 5/8 of their maximum length.

REVISION
Below is a more complete sprite size overview by Michael Blunck:

source

An even more advanced images is below:

source.