Wondrous Item, common
A child's bath toy shaped like a duck with runes all over it that don't show unless being used. It doesn't change size unless placed on a very specific type of surface and its size changes depending on what surface it's placed on as well as the terrain; it won't get as big placed on a dirt floor if said dirt floor is inside versus outside. If placed in water that's not inside a bathtub, it will either grow as big as the water allows (to a point; placed in water that's in a harbor or out in the sea and it can become as big as a ship) or multiplies. If the latter, each new duck has become its own Rubber Duck of Randomness
To add as I've found I can't edit once it's published: for the 'If placed in water that's not inside a bathtub, it will either grow as big as the water allows (to a point; placed in water etc) or multiplies': I would suggest using a d20 for this (or any other dice that has an equal number of even and odds). Evens will get you one option and odd another. Alternately, the lower half of the amount (10 and below for a d20, 1 and 2 for a d4, etc) will give you one option and the bigger half (11-20, 3 and 4, etc) will give you the other option. Use one of the bigger dice (d20, the d10 that's for the hundreds, etc) for how many it'll multiply into if you land on the multiply option. Placing a Rubber Duck of Randomness in a glass of water or any other drink will have the same effect as placing it in a bathtub. Or it might shrink. It's random like that.