What size is your dog?
Will it be your dogs only toilet or just for nighttime, when you're not home etc?
How much space do you have and where you will place the toilet?
These measures are approximate, and to use as a guideline only.
Choosing the right size toilet for your dog is important and will make your dogs new loo experience a more comfortable one..... and it will also reduces the risk of "misses".
Monitor how your dog usually toilets? Is he a squatter, a circler, a leg lifter? He may need a little more space if he tends to circle or is a constant leg lifter.
Many dogs also like to circle prior to toileting so giving them enough space to circle will make their new bathroom a much more natural experience. For boys who like to cock their leg I find most will revert to the low leg lift. If your boy is a leg lifter avoid placing your PupGo up against the wall until he gets the hang of it. If he has trouble peeing without lifting try placing a small target like a small used soft drink bottle filled with sand or similar in the center of the mat, don't place near the edge, you want him to be aiming for the centre, don't make it too high either as you want that pee to land in the mat.
Main or only toilet
If the PupGo is going to be your dogs main toilet option then I always suggest bigger is better. If your dog measures near the top range of smaller PupGo 55 and you have the space I would definitely choose the larger PupGo 80. You want your dog to be comfortable using his toilet so making sure he has enough space to do his business many times a day is a must. For multiple small dog families, the larger capacity Pupgo will also allow for less need to clean during the day.
If your dog will be a casual user eg. just for nighttime or when you are out during the day, then the amount of space is not as critical, as long as he can comfortably get all four paws on the tray to do his business allowing room for the poop squat.
Consider where you will place your dogs toilet. How much space do you have? Going large is great for your dog to give his a good patch to toilet on but if space around your PupGo is tight then the confined feeling may not be so comfortable for your dog.
Best placement is to have at least 3 edges free so your dog can walk on and off easily without restriction or the need to turn around or back out (this could result in messy feet).
Areas away from main living rooms like the laundry or bathroom are great places for you indoor toilet. It gives you easy access for cleaning and waste removal plus any "misses" can easily be wiped up on hard floors.