> There is no method in RMagick that takes an image and a file size in 
> bytes and returns an image that, when saved to a file, produces a file 
> close to or no larger than the specified size. The file size depends on 
> the image format, the compression level, and the image itself.

There are tools that can estaminate size of file after compresion - but 
they'll never give you exact size without actual compression.

So .. you take script, 20-30 simple images, take each one, start with 
some quality - and incrementally decrase untill size fits, then you take 
mean and min from qualities.

now you have 2 qualities - one that 50% of images should fit with size - 
and second that almost all should fit.

that's assuming that your surce images have simillar resolutions.

greets