Can you play a MIDI keyboard without a computer?

While MIDI controller keyboards don't have any built-in sounds, some MIDI keyboards feature a 5-Pin MIDI port which would allow you to use a compatible sound module as your audio source.

Does a MIDI keyboard need an audio interface?

A MIDI keyboard will directly connect to your computer via USB and while you will be able to play virtual instruments, you may encounter latency. Latency is the amount of time your computer takes from receiving the MIDI note information from the keyboard to producing an audible note. The most common way to combat latency is by using an audio interface, which features built-in processors to help your computer to play low latency audio.

Should I get a 25 or 49 key midi keyboard?

It's down to your personal preference. If you're looking for a keyboard you can throw in a backpack, 25 keys is the way to go. However, if you're looking to play with two hands, a 49, 61 or 88 key MIDI keyboard would be more beneficial.