Depends on the ratio of the DVD. Mostly it's on the cover: 16:9, 4:3 etc.

The picture you have is 750x600, that's 1.25 (750/600). If you do 750/1.25 you get 600...

If the ratio on the cover is 4:3, you have to do: 750/1.3333 = 562.

4:3 = 1.33
5:4 = 1.25
16:9 = 1.77

there's also 1.66 and 2.35 but I have never seen them.

You can access this in irfanview by control-r. Note the checkbox 'preserve aspect ratio'. This checkbox has to be off.