How can I find thecapacity of an older model dryer?
I tried googling and found this: 

Question about 79891 dryer 

which is the identical question you are asking. The answerer stated that the full model number is 110.87980100. Based on that and searching again I get:

Question about 110.87980100 capacity  

which states for model 110.87980100 the capacity is 7.0 cu ft