I'm thinking of doing a nano FOWLR, which some day might convert into reef. What is the smallest size I can get away with? What would be a good recommended size? Any advice is greatly appreciated, thanks.
As small as you want really. I've seen people use like half gallon glass vases as pico reefs. If you want fish though you really don't have any options until you get to 10 gallons and even then you are limited. Also you need to remember the smaller body of water the faster fluctuations will happen in temp/chemistry. Being your first tank I would reccomend as I do with everyone else a 20 gallon long or a 29 standard if you don't wanna go with something larger (best tanks imo are 40gallons+)
Fish only if you want anything interesting nothing less than 30 gallon but that still limits you a lot you will also find it difficult to keep small reefs due to how difficult it is to maintain parameters why do you need it small?