She refers to his endowment as "impressive", not "massive". Who knows? The older son, David, was called Donk (as in Donkey Dick) by his siblings.
On the contrary, having read the book. Jones spoke of Jack Cassidy as being a "sex God," her ideal/perfect man, etc. a number of times. Among other comments, she described him as "extremely well endowed "--describing it as a "blessing"--and as having a "giant endowment."
Specific quotes where she comments about Jack's penis size, and her sons' sizes:
Losing her virginity to Jack and their sex life:
"He was a great lover, that night and every night afterward. He could go on for hours, have two or three orgasms, then wake up in the morning and make love to me all over again. He was inventive and extremely well endowed (a blessing that all his sons, in particular David, inherited)."
Commenting on Jack's size regarding a story Jack told her about what he said was a sex encounter with Cole Porter--Jack had revealed himself to Porter (at Porter's request; others--she cites a Truman Capote story about this--say Cassidy was involved with Porter more than once), and teased him with his penis, asking Porter if he wanted it:
"Jacks endowment was so vast, so desirable, that I had no doubt whatsoever about Cole Porters answer."
About Jack's and David's size:
"David also had something else in common with Jack: a giant endowment. Davids brothers called him Donk, for Donkey, and Jack would joke, Where did you get that? Youre bigger than me, which probably didnt help their rocky relationship."
Sounds like Jones is particularly interested in big ones and size. Mrs. Partridge a size queen? (which I don't mean in a derogatory sense, seeing nothing wrong with someone having that preference) Maybe she's a member here.