Some of my pixs are pretty big, but I use postimage and have never had a problem. You may fill up your quota on the hosting site faster with big photos but I have several score of them on postimage. And, while the FSM forum software reduces size to fit the forum format, if a viewer clicks on the image FSM directs to the hosting site and you see a big version of the image, so bigger photos do appear more detailed.
Also, when selecting the image during the posting process, you do not need to fill in the size- there is a default value which works fine.