Is this radiator too small?
First of all I assume that the radiator is functioning properly and gets nice and hot(215F for steam) If not than it is not a problem of sizing.

To figure out if this radiator is appropriate for the given room we need to calculate how much heat it emits. It is all based on its surface area. This radiator looks like it could be National Aero Four Tube radiator or American "Corto" Radiator. Those radiators with four sections and in 26 inch size both have 11 sq ft of radiation. 1 sq ft gives off 240 BTUs at 215F. So this radiator emits 2640 BTUs.

Now just calculate how much heat this room of your home looses and you can see whether this radiator is adequate. There are quite a few online tools available to calculate heat loss. None of them will be too precise without blower door test, but you can probably get a ballpark figure. I did a quick google search and this seems to give you necessary steps to do a basic heat loss calc:
http://www.pexuniverse.com/content/calculate-heat-loss

There's also software available to help you. For example HeatCAD http://www.avenir-online.com/avenirweb/HeatCAD/HeatCADHome.aspx has a 30 day trial.

It will probably make sense to compare heat loss numbers with other rooms in your house that are known to heat well.

Once you know for sure what is causing the room to be cold, you can remedy the problem. You could either increase the radiator size or spend some time insulating/tightening the room(this would be much cheaper and more efficient)