Aggregate or summary?

You cannot use summary here because it means something else. Other than that, it's mostly down to preference.

Although aggregate is technically correct, I would only use it when describing the size of a new file created by merging the two original files - not the result of summing up the two original sizes. I would also prefer using a narrower term (for example, aggregate functions include not only sum but also count, average and more).

I normally label the result of summing up several values as total. When referring to the total in a sentence, you could also use speak of the combined size of the two files.