Will I get a gasgas in a box trailer height is 4 foot 6 foot long

Yep, but your'll have to turn the front wheel a tad..

I have a land rover with a 6ft bed and I can get 2 bikes in head to toe.

Should be no problem. Mine is 6 ft x 4 ft x 5 ft high and I can get 2 full size trials bikes in with front wheels tuned to the centre.

Cheers

John

This is not a box trailer I know, but it is a bog standard small 4' wide, 6' long garden trailer. Rather than turn the front wheels, the rear tyre extends (and is held tight) by the tailgate.