I'm planning to buy some equipment (plastic boxes) to store the Descent stuff, currently i have only the Base Box.

Question: Are there any terrain tiles in an expansion which are bigger in size (and if yes, how big) then the biggest terrain tile in the base box?

Base Box max. is 8 x 6 squares.

In LoR there is an 8 x 8, but that is the biggest.

In D1E, they did eventually get up to 10x10. Not saying the same is guaranteed to happen in D2E, but if your storage solution can fit it without adding too much cost, it couldn't hurt to be prepared.