How to change a spiders size in minecraft?

Spiders of minecraft have a fixed size. In order to change that size, you would need to either download a mod that does so, or make the modifications to the game yourself.

To my knowledge you can't change the size of spiders in vanilla minecraft.

It is impossible as of 1.7 to change a mob's size from small to big or vice-versa. The sizes are fixed and cannpt be changed. The closest you can get is to have a giant zombie(aka Giant), but the Giant was merely a magnified zombie, originally intended to be implemented, but was cancelled due to it being too OP. You can still spawn it in with command blocks.

A giant spider sounds like you're trying to make a boss. You can stack multiple spiders together, or have a spider with an insane amount of health.