Here's what I'd do were I in your shoes:

Announce that any future submissions that exceed the dimensions and file size will not be disqualified. The Monthly challenge rules are in the forum banner, so they're quite accessible to anyone. If you want to go all out (due to the new popularity of the challenge, you could make a page in the backroom with the rules that's easier to read.)

I'd also just downsample the 6 entires. So, if you have an image that's 1000x600 pixels, I'd make it 800x450 for them. That would put the entry at the maximum resolution for its dimensions, and it would also probably eliminate the file size problem.

If you have a file that's 800x600 and still weighs in at 207kb, It's so close, that I'd let it slide, at least this time. It's not as big an issue anymore because of your snazzy new page format for the Challenge. File size was more important when all the entries were in one thread and load time was sloooow, but now that each entry loads individually and the thumbs are pretty small it doens't matter as much.

As for people who might feel slighted by maximum pixel size, it's unfortunate indeed, but think of it as a challenge! If this were a brick and mortar gallery, I'd get rid of size limits too, but we're online, and over a third of web users only have a screen res of 600x800!

P.S. I'm tinkering with posting in html, so forgive me if this post looks bad!