We can specify thickness which only makes it taller but not length

Try duration tag

Yar, duration is what you're after. Valid values are 0 - 10. 10 will result in a beam that takes a while to dissipate. It can actually be made to be a pretty cool effect.

I don't see a cap of 10 after a quick glance at the code;

Note that for LaserCannon the length is