I read the book: Essential Genetics and Genomics It has a table summarizing the properties of the "typical" human gene:
It has a gene feature
Size of internal exon, what internal means in this context?
When I searched for it I found mentions about external exons, but found no definition.
I strongly suspect they mean:
External exons are the first exon right after the 5' untranslated region and the last exon just before the 3' untranslated region and all the other exons are internal exons.
As turned out this was almost right, but had a mistake which is wonderfully pointed out in the accepted answer.