A voltage drop calculator may be useful to find the gauge of wire you want to use. Usually you want less than a 3% voltage drop if possible. *deleted previous comments about amperage, had a major brain fart*
http://www.calculator.net/voltage-drop-calculator.html
Also, look in your owner's manual for what extension cord you need to use. Most welders have this information in their owner's manual because for these beasts, it is important that they have the proper gauge extension cord so you aren't popping breakers, starting fires, etc. I just bought a Hobart Handler 140 and it had a small chart that listed lengths of extension cords and what gauge wire it should be. Very useful!
My Ironman 250 is a few years old now... and a bit different then a current model 230.
but I opened up the owners manual just to see what it had to say on the subject (thanks for the reminder)
While there is no verbiage discussing extension cord... it does have a section on power supply.
Yes, at 230v it pulls 35 amps as already mentioned. The manual calls for a minimum of 10AWG wire and a Max Recommended Input Conductor Length In Feet" of 94 feet'

So the answer was there all along... in digits on the same laptop I typed the question.
Add to that data a few calcs for voltage drop... and clearly 8/3 SOOW is more then enough for a 30-50ft extension cord.
Now to find plugs and receptacles for it.
Cyberweld has a 230 Volt Extension Cord features 25' or 50' of 8/3 power cable with lighted NEMA 6.50 (pin) male (plug) and female (receptacle) at each end. at a good price all things considered... but it uses a NEMA 6.50 setup.. and my welder and 230v outlet in use already are NEMA 10-50.
So I have to change the in use plugs to NEMA 6-50 or make a cord using NEMA 10-50