There are a lot of MIDI keyboards out there as there are the companies that make them - Yamaha, Roland, Korg and Casio among others. Having one of your own is a big must have if you're a musician - amateur or professional. It doesn't take rocket science to buy one but it does require some mental considerations before you get the one you want to use.
There are many different types of units out there - from the big piano types to the little controller ones. Nowadays most electronic keyboard have MIDI capability so your choices have gotten even bigger. If you're looking at a portable keyboard the thing you want to focus on is transportability. Just because they called portable keyboards doesn't necessarily mean they are all easily transportable. That's where their size comes in. By size is generally meant the number of keys that they have.
88 keys means piano keyboard. These MIDI keyboards tend to be bigger and heavier. They are still portable but they can be cumbersome, especially if you're always on the move. Set up can also be a problem depending on where you use it. If on a stage then you may have not have a problem with space but if it's at home then it would be extremely difficult to squeeze something a couple of feet long in front of your computer next to the keyboard. The same goes for the 76 ones.
Then there is the smaller ones - the 37's and the 25's. They are small and lightweight and thus very portable. For the most part many are satisfied with the smaller units because you can still do all the things you can do with their bigger siblings but in a smaller package.
That being said the smaller ones (37's and 25's) tend to be MIDI keyboard controllers rather than actual piano style keyboards. That means they are used more to control other MIDI compatible instruments and creates music out of them. These machines have no built in sound system of their own meaning you won't hear anything when you press the keys. You will need a sound card and audio editing system to hear anything - your computer will do the trick.
Then there's the knobs and controllers to think about. Most units will have a range of buttons, knobs, sliders, faders and a LCD screen to boot. The bigger the unit the more widgets you can get. Mesh this with good Digital Audio Workstation software and you can produce just about any sound instrument you like. Of course the bigger the MIDI keyboard and the more capabilities it has the higher the price. Many recommend a simple and small keyboard as it can pretty much handle what a majority of musicians want. That is to say it doesn't matter if you only have a 25 key unit as long as you are able to maximize its use then it's the right unit for you.
You also need to consider the learning curve necessary to make full use of all your unit's capabilities. This is especially true if you're a beginner getting your first keyboard. Finally there's the feel. If you feel right about the keyboard then you can make great music on it.
