I'm not going to try to recommend a particular controller, since there are a lot of variables and personal preferences to consider, many of which can be loosely lumped under your least-important criteria (size):
- Number of keys? (88, 76, 61, 49, 37, 25, etc.)
- Shape of keys? (boxed piano-style or flat organ-style)
- Type of key action? (weighted, semi-weighted, non-weighted, etc.)
- Size of keys? (standard, mini, or micro)
- Type of aftertouch? (polyphonic, channel-only, or none)
Those factors alone can be the make-or-break considerations for a serious keyboardist-- and while they might not be as critical for the rest of us, making a few decisions about those criteria can be very helpful in narrowing down your choices.
Then there are factors such as the number, types, and even the positioning of the other controls-- pitch bend wheel, modulation wheel, knobs, buttons, sliders, pads, etc. For instance, maybe you prefer to have the wheels to the left of the keyboard, or maybe you don't mind if they're set above the keyboard? Maybe you want to have no fewer than nine slider controls that can be set to "drawbar mode" (i.e., the "0" position is all the way up rather than all the way down) for controlling virtual drawbar organs? Maybe you need a lot of knobs for mapping to the controls on soft synths? These factors might not be as critical, because if your choice of keyboard controller (i.e., based on number/type/action/size of keys) has only a limited number of other controls, you can always buy a second controller that has all the other controls you want but is lacking the type of keyboard you prefer. Or perhaps you prefer a single all-in-one controller rather than assembling a small arsenal of controllers?
As far as brands, if I were shopping today for a keyboard controller for myself, I'd probably lean toward something from Akai Professional, Native Instruments, or Novation (listed in alphabetical order, not order of preference). I don't have any experience with any of those brands as far as keyboards are concerned-- the only keyboard controller I have (not counting actual keyboard instruments) is an old M-Audio Axiom 61-II-- but I do have non-keyboard controllers from Akai Professional and Novation which seem to have sturdy build qualities. I'm not familiar with Nektar yet, and I might also be interested in something from Arturia that would be designed to work with their V Collection.
And you shouldn't rule out companies that make actual keyboard instruments-- e.g., Korg, Roland, Yamaha, etc.-- as they also make controllers.
Sorry if I haven't been of much help with specific recommendations, but hopefully if you start making a few decisions about some of the factors then your choices will be narrowed down considerably-- and then making additional decisions from there will help narrow the choices down even more, until you're eventually left with a choice between just two or three.
