These LED's are 12V SMD 5730 size and with the two rows of them throw significant light on the print bed without adding any mass to the Kossel hot end.

12V LED's are the obvious choice since you don't want to draw any more 5V power from either the MEGA's USB buss or the MEGA's skimpy 5V regulator. Stick with 12V so it will run off the 20A or 30A 12V supply for your printer.

The slot the LED strips fit into will hold any LED strip that is 10mm wide, but if they aren't 5730's they will emit less light. The strip LED's come with self adhesive backing but I don't peel the backing off, just slide them in the slots, wire them up, good to go.

Watch the polarity of the LEDs when soldering them. I join two banks of them together (one at the base of the X and Y towers; you probably don't want it on the Z tower since it would shine into your eyes).

Power can be right at the 12V input screw terminals on the RAMPS 1.4 PCB.

Or you can wire them up to the FAN terminals - D9 - on the RAMPS board if you're not using a fan. This way you can control them with GCODE commands. Again, watch the polarity if you wire them to D9. Note, if you wire them to D9, it is a PWM control so not only can you control the brightness, they will slightly flicker because of the PWM. Wire them straight to 12V if that bothers you.

I have mine wired to D9 and come on at the start of the hot end heating phase and turn off at the completion of the print. In Slic3r I have start and end GCODES set:

Start GCODE:
M106 S255 ; LED lights ON

End GCODE:
M107 ; LED lights OFF