Can Amazon Milk Frogs be placed in a long aquarium?

I think it really depends on how many you want to keep, I know that for 1-2 frogs a 29-gallon long is really the best size (although ideally, bigger is always better - so something like a 40-gallon breeder could even provide more space for them to roam and grow in).

They do need water, and it really depends on how this slope is and what you may mean by halfway in the tank. Does the water start at the 50% middle line, or does it start a tad further? You can always have pockets of water/mini-ponds within the tank for them to use which is common with people who can build them with rocks/decoration and still allow for them to have a slope-like feature.

When it comes to Amazon Milk Frogs, the more space you can give them the better it will be for them. I know the recommended size for 2 - 4 adult Milk Frogs is a 29 gallon aquarium that measures 30 inches in length, 20 inches in width and 18 inches high. More room is of course recommended if you are able to but that would be the ideal numbers to go from depending on how many Milk Frogs you are wanting to keep in the tank.