How can I take screenshots of full threads?

I downloaded an addon for Firefox but whenever I take a screenshot of just the main page of any board it is like, 3.5 MB. What program can I use to take images of full threads where they won't end up being  3.5 MB?

Also, I'm new to 4chan BBS, I'm usually on the imageboards, so if this is the wrong board I'm sorry.

Thanks.

I use Screengrab. And no, I'm not worried about it taking up space on my hard drive, but whenever I try to upload it to 4chan or an image hosting site, it always doesn't let me or shrinks it down to where you can't read anything or see anything really.