Hello! I'm getting a 74 DT250 back up and going but every flexible line on the bike has rotted off. I have the vent lines for the carb (new carb) but that's it. What sizes are on the bike? I plan to grab some different sizes of Tygon and make new lines. I'm sure I could just size them off the hand but I figured one of you would maybe know the exact sizes I need. Off the top of my head: oil tank vent, main fuel line, oil tank to pump, pump to cylinder, crank vent and I think that's it? Let me know if I'm missing something. 

my unqualified experience in general with Tygon for anything other than vents (really good) is "just ok"
I also have used automotive hose with fiber for fuel with an external filter and like it better than Yamaha lines.
the Yamaha lines (Partzilla) are the best for autolube IMO.  I don't know why, perhaps more carbon black or mers in the polys

 

I have an original parts manual for the 1974 DT250A and DT360A (Euro version, but no difference for this purpose) and the dimensions for two of the pipes are shown:-

Fuel tank to carb - internal diameter of 7mm, length 200mm.
Oil tank to pump - internal diameter of 5mm and length of 675mm

Dimensions are not given for the oil tank breather (probably same internal diameter as line to pump) or the pump to cylinder feed line.

I hope that this helps.