Peccaiv is right use a ruler.
Apart from this question would fair better in the caravan/motor home section. If it's the older type translucent roof light remove the screen and measure internal measurement width and length of the frame these are usually square on UK caravans, the toilet one being the smaller of the two. If you only need the cover they will know what size by the frame measurements.
BB
January 2010
Much the same as measurement in a static caravan or in a house...
A ruler is best - graduated in imperial or metric - you can do area LxW - L and W or height from the floor.
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