You've been reading! There are tons of good threads on the HW website also. Put some keywords in the search box at the top of the page.
Portion size makes little difference if it is foods that contain sulfite. If you are over your tolerance level, which could be one can of soda, you will react. The website I used to find out how to calculate how much is in certain foods is http://www.learningtarget.com/nosulfites/index.htm
What is your reaction? If you have a "true" allergic reaction you have to be especially careful. I have a "sensitivity" but still have to be careful because my reaction is tachycardia which has sent me to the ER all too many tmes. Some people have migraines.
If you have a severe reaction you shouldn't experiment to see what your tolerance levels is, but you can keep a good food journal and make a pretty good guess - using the calculations in the No Sulfites website. (click on the url I posted above.)
The main thing is to avoid processed foods, including things like commercial bread, even at the local bakery, unless you have a bakery that uses no preservatives. No sodas, no wine or beer, very little Italian food unless you make your own sauces without onions and garlic. I don't tolerate pork at all, but can have a little beef. If I avoid foods with high sulfite, I tolerate a bit of milk chocolate. (Forget the nice dark chocolate though.) But keep reading. You're doing well!
A good, experienced allergist can help. If s/he says there's no such allergy find another one! My 3rd one has been a big help.
Watch your vitamins too. Biotin has a lot of sulfite. I don't take anything extra for lowering sensitivity, not sure if it does much.