How much does one Spanish chorizo weigh?

Sausages normally aren't graded by size/weight, especially something like chorizo that comes in a very wide range.

I'm guessing that most medium sized Spanish chorizos ranges somewhere between 70-80 grams.

Concerning those recipe instructions the point is to use a medium sized one that is widely available. That's basically the same with any ingredient that isn't highly specific about size or weight. Just use what is typical in the major supermarkets in your area.

Remember that in the 19th century home cooks didn't work from recipes with careful measurements. "One chorizo" meant one piece -- whatever you had on hand. This would also imply that if you wanted more sausage flavor, you'd pick a bigger one and if you wanted less sausage flavor you'd pick a smaller one because back then they were pretty much all handmade.

It's a judgment call for the individual cook based on his/her taste buds and those joining their dinner table. In my kitchen, invariably I would probably go for less meat thus a smaller chorizo. I know many people -- especially the average American -- would favor more generous portions of meat and reach for a larger sausage.

It's really your call.

Gary - seems to be a great variety of type and size out there.
See a site called “Jaminarium” [ham-store] for more info, recipes and products to buy.
Probably go for quality vs size, get the best you can afford and - if necessary - adjust the other recipe quantities to suit the chorizo you get.