The owners/manufacturer can confirm this but I'd wager the Downtube takes ETRTO 406 sized tires - aka "20 inch."

These are arguably the MOST common tire size next to MTB 26 inch and 700c - making them VERY easy to source. Tons of options for 406 sized touring tires. I'm running 100psi Schwalbe Marathon 20x1.50's. (40-406)