I would say a 30 or a 40 cu ft tank, but definitely not a 13 or 19. Depends what you want to put in it.
I have both 30 and 40 cu ft tanks, the 30 is my usual pony and I tend to use two 40 cu fts for deco, one for 50% and one for O2.
My other half uses the 30 cu ft for O2, and a 40 for 50%.
30 cu ft is way too small for a mix like 50%, but is fine for moderate deco using O2. In reality, you won't notice any difference between carrying a 30 or a 40, so the suggestions to go the larger tank will give you flexibility.
I have both 30 and 40 cu ft tanks, the 30 is my usual pony and I tend to use two 40 cu fts for deco, one for 50% and one for O2.
My other half uses the 30 cu ft for O2, and a 40 for 50%.
30 cu ft is way too small for a mix like 50%, but is fine for moderate deco using O2. In reality, you won't notice any difference between carrying a 30 or a 40, so the suggestions to go the larger tank will give you flexibility.