A: If I understand your meaning of “lay flat”, yes, this hose is quite friendly and compliant in a wide temperature range, and will lay completely against the floor when stretched out, and is easy to coil by hand for compact storage.
A: Yes this hose lays flat. It is very supple for it is rubber. Coils up easily.
A: Inside diameter. The inside diameter is 3/8 inch, outside diameter is just over 1/2 inch.