WHy did you need a keyboard tray?
The monitor does look way too small for the cabinet. But it is fine assuming you are only concerned about playability and not about looks. That cabinet would hold a 25" monitor with a minimum of effort, so I am sure you could fit a larger one than that 14" model.
Since this is a mame cabinet I'm going to want to be able to work with the computer from time to time. Most of the time I'll be using a fronted designed for cabinets so I won't need the keyboard or mouse. I could always get a wireless keyboard. But because of the tight space I'm working in I don't really have any place to put even a wireless keyboard, and I dislike typing on my lap. So the tray seemed like a good compromise, some other cabs have them too. Besides it slides out of the way and is high enough, right under the controls, that you don't see it. There are also a few games for which the arcade controls I have won't be very good for these I can still use a mouse or keyboard.
About the monitor, I'm thinking of putting a 20+ inch television in there. I'll have to see what fits, I might have to lower the monitor shelf a bit.