The size of the decoder is important only to the fact of whether or not it will fit in the locomotive. As long as the decoder will fit inside the locomotive, and it provides enough current for the motor and all the functions you'll be using, all other features are purely personal preference.

If a decoder is made for a specific locomotive, there is nothing to worry about. The manufacturer has insured that it will fit in the designated spot. Likewise, if your decoder vendor has sold you a decoder for a specific loco, he is indicating that that decoder will fit. However, there are many specialty locos made that were or are not a popular item. In this situation, there probably isn't a decoder made specifically for it and your dealer may not know about that loco. Often times your dealer can recommend a decoder based upon your description of what the inside of the loco looks like, but other than that you're on your own for determining which decoder will fit.

While the decoder scale designation is an indication of the size, it does not indicate whether or not it will fit in any particular loco. Manufacturers usually also publish the physical size of their decoders for you to determine if they will fitt. However, visualizing whether or not a certain measurement will fit in a particular location in your loco can be iffy, at best. For this, we have two suggestions.

You can make a small block of wood or plastic that is the exact size published for the prospective decoder. You can then install that block in the loco where the decoder will go. If you can get the shell back on all the way, then the decoder will certainly fit. What this does not take into consideration is the contour of the decoder.

For another method to find how much head-room (the amount of room between the chassis and the shell, where the decoder will go) there is in a loco, you can use a piece of Play-Doh® (a child's modeling compound). Place a piece of Play-Doh on the chassis where you want to put the decoder. Install the shell, making sure it seats all the way down. Remove the shell. You can now measure the thickness of the Play-Doh where the shell has squished it down.

While manufacturers provide the physical dimensions of the decoders, these dimensions are usually the maximum for any given dimension. This does not take into consideration that the maximum thickness may be a capacitor or other component that sticks up in only one place, and the rest of the decoder is much thinner.