Official spec is 40 liters / 10.6 gallons:
InsightCentral.net - Encyclopedia - Honda Insight Specifications
I would not be surprised if it actually held a little more.
Regarding the LEDs, did you get white or soft white? I find that the 6000K (?) color of most LEDs is a bit harsh for me. The
Zone Tech warm white LEDs that I bought for my car had a lot of variability in light color, but are overall not bad.
I recommend something like this for your dome light:
http://www.amazon.com/White-Light-Pa...ilpage_o06_s00
I used SuperBrightLEDs.com for my hatch festoon and tail light bulbs:
https://www.superbrightleds.com/vehi...=977&year=2000
^ They have a reputation for having only top-notch LEDs (no cheap Chinese nonsense)