What is the problem you're trying to solve
Compose file format 2.3 allowed to use tmpfs.size with bytes
Compose file format 3.6 only supports integer with constraint minimum:0

To apply our goal to support them all, we need to redefine tmpfs.size so both are valid

Describe the solution you'd like
a mix of both, but if too complex to be expressed by json schema, better adopt the 2.3 ["integer", "string"] definition

Additional context
docker/compose#8696