My yoyo (Pakistani dwarf) loaches grew to around 5 inches for the males and 6 inches for the dominant female (subordinate females were smaller and similar sized to the males). They were kept in a 4x2x2ft aquarium with lots of plants and an undergravel filter. They went down the filter uplift tube during the day and lived under the filter plates, coming out for food and when it was dark. They basically had a 2 story tank to play in with 8x2x2ft area. I had about 20 of them in the tank and they bred.
I gave my friend 10 for his tank (6ft long x 14 inches wide x 18 inches high) and his dominant female hit 8 inches long and the males were easily 6 inches long.
Whilst these loaches are sold as a dwarf loach, there seems to be some variation in how big they get. Some people say they don't get bigger than 3-4 inches but my experience is the ones I had got a lot bigger, and my friend's fish got bigger again.
The smallest size tank for a group of 6 of these fish would be at least 3 foot long and 4ft+ would be better. They also need to be in groups of at least 6 and 10 or more is great.