MicrostockGroup

There are many programs that can batch-resize pictures: XnView is one.
The problem is that resizing is based on width or height, not megapixels. But it's easy to do some calculations, especially if your pictures have mainly the same ratio, e.g. 4:3, 3:2, ...

But the only site that allows upsizing (from 6 MP to 8 MP) is Alamy - all other sites explicitly forbid upsizing or allow a max 5% increase; all you will get is being banned.

Windows 7 can do it I think.
In "my computer" (file browser) you can add a column that gives you file resolutions and then you just sort by rez.

ACDSEE
I stopped using ACDSee, since their TIFF browsing is dead slow, but what you want is built in the software. When viewing the images, select list and select some custom columns that fit your needs. Then sort by the choice, by pressing the column header.