Buying a light upgrade need help

[ddek1]

ive decided i want to upgrade my light fixture but have no idea what type to go with. im leaning towards t5, should i go 2 bulb or 4 bulb? i have a 29 cube so should i go with the 18inch or 20 inch?

I honestly have the slightest idea what to do...

 

[Jabibi]

I would go with a 4 bulb.