Length across back

i'm am desperatly looking for a tent but one thing gets in my way. the length of the pack. i measured the length available across the back of a bike (overlapping the panniers and made it out to be a little over 50cm. most tents come in packs that are longer and the few tents that are compact are expensive. also i wanted a stuff sack and they don't come under 50cm unless the capacity is ridiculously small.
so what is the length you recommend is good to go across the back of a bike?

I cary our tent which is 55cm and my wife cary's the mattresses, 63cm. The mat's overlap the bags by a few cm's, no problem.
Pictures on our page.
cheers

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