A quick sketch can give the photographer a 'better than guesswork' chance of supplying the images you need.
So, you need a girl on a bridge by a river, with some tall buildings in the background. You need it, for your main image, to be 4:3 portrait, but you also need to be able to re-crop for a banner...
'scuse the 'fine art' [I'm a photographer not an artist] but this is about all the illustrative detail required...
so this is what they need to shoot...
Give them all 3 crops & they can immediately see what you need. Keeping the illustration intentionally vague other than positionally allows them to place their own artistic stamp on the imagery, which is presumably why you chose that photographer.
BTW, "step back a bit" doesn't work as an instruction, because the photographer's conscious framing of a scene will already include elements such as background compression - which is in itself a feature of distance to subject:background as a ratio, combined with lens length to bring that chosen distance to best fill the frame.
"Use a shorter lens" would be the objectively 'best' instruction, but you have to allow that the photographer will already know how to do this.
"Please leave us a little extra crop" might be the most succinct way to get the message across - but personally I'd like to see that initial descriptive paragraph + those 3 simple drawings too.