Student or Learner
.A two gallon's jar.
.Two gallons' jars.
.A stone's throw.
.Two stones' throws.
.A two span's room.
.Two spans' rooms.
Could you please tell me if they are OK or not?
"A stone's throw" is OK. Not the others.
We can have a gallon-jar and a two-gallon jar.
'A stone's throw' is a fixed idiom. We never speak of 'two stones' throws'.
The fifth and sixth are meaningless.
A two-gallon jar. I'm not even sure how to fix the last set. What does it mean?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.