I am trying to make a 1/48th scale diorama but i don't know what size of trees I should put in and it is a winter theme

Well - just converting a few scale feet to 1/48 you'd get
40 feet = 10 inches
30 feet = 7.5 inches
20 feet = 5 iches

6 feet = 1.5 inches- male size benchmark

So depending on the overall size you're looking for you need something I'd say around 7 - 10 inches tall.

Just put the figure next to the tree and see if it looks about to scale thickness wise.

that is a good scale to put it at. but there also some scale changers out there that can be easily downloaded. just type teh size in and it will tell you the scale.
Graeme