The memory requirement deal is something I've looked into before.The memory required for running the basic software itself is one thing, while the memory required for media resizing comes with a more standard and predictable formula that binds all platforms.

Here's an approximate chart of how much memory is required for scaling down different sizes of photos. Test images were scaled down at 1 Mpx intervals and saved with JPEG quality 90. What they are scaled down to is not that important, what counts is how big a source image is loaded into server memory.

Dimensions: 640 x 480 px (0.31 Mpx)
Image size @ q90 JPG: 193 KB
Memory needed: 1735 KB (1.69 MB)

Dimensions: 800 x 600 px (0.48 Mpx)
Image size @ q90 JPG: 291 KB
Memory needed: 2646 KB (2.58 MB)

Dimensions: 1024 x 768 px (0.79 Mpx)
Image size @ q90 JPG: 458 KB
Memory needed: 4262 KB (4.16 MB)

Dimensions: 1100 x 900 px (0.99 Mpx)
Image size @ q90 JPG: 611 KB
Memory needed: 5336 KB (5.21 MB)

Dimensions: 1560 x 1280 px (2 Mpx)
Image size @ q90 JPG: 1057 KB
Memory needed: 10645 KB (10.4 MB)

Dimensions: 1910 x 1570 px (3 Mpx)
Image size @ q90 JPG: 1608 KB
Memory needed: 15929 KB (15.56 MB)

Dimensions: 2210 x 1810 px (4 Mpx)
Image size @ q90 JPG: 1990 KB
Memory needed: 21209 KB (20.71 MB)

Dimensions: 2470 x 2024 px (5 Mpx)
Image size @ q90 JPG: 2277 KB
Memory needed: 26479 KB (25.86 MB)

Dimensions: 2700 x 2224 px (6 Mpx)
Image size @ q90 JPG: 2510 KB
Memory needed: 31781 KB (31.04 MB)

Dimensions: 2920 x 2400 px (7.01 Mpx)
Image size @ q90 JPG: 2654 KB
Memory needed: 37071 KB (36.2 MB)

Dimensions: 3130 x 2560 px (8.01 Mpx)
Image size @ q90 JPG: 2833 KB
Memory needed: 42370 KB (41.38 MB)

Dimensions: 3300 x 2730 px (9.01 Mpx)
Image size @ q90 JPG: 2959 KB
Memory needed: 47624 KB (46.51 MB)

Dimensions: 3500 x 2860 px (10.01 Mpx)
Image size @ q90 JPG: 3038 KB
Memory needed: 52902 KB (51.66 MB)

This was counted with a basic formula by simshaun from a DevShed thread — of course different platforms can add to the memory requirement by doing something complex with the images loaded into memory. However, with the figures on the table, it's easy enough to calculate a baseline for how much your server should be able to handle.

Of course if you have a bulk uploader, and if you're uploading bigger images in bulk, you will hit the ceiling much quicker, as it all counts into the memory allowed for a single script execution.