Green’s Dictionary of Slang

a diminutive person.

1914		T.A. Dorgan in Zwilling TAD Lex. (1993) 42: You — half portion I oughta.
1920		Wodehouse Coming of Bill 59: He certainly is a kind o’ half-portion, ma’am!
1925		D. Hammett ‘Tom, Dick, or Harry’ in Nightmare Town (2001) 240: He was such a little half-portion of a man.
1927		M.C. McPhee ‘College Sl.’ in AS II:2 131: He announces his opinion of the social and intellectual gifts of a fellow student in terms like: [...] ‘a half portion,’ ‘a humdinger,’ ‘a good date,’ or the person may be ‘just rare’.
1931		D. Runyon ‘Tobias the Terrible’ in Runyon on Broadway (1954) 114: This half-portion here is the desperate Twelve-Gun Tweeney?
1967		Wodehouse Company for Henry 130: Even when calling her a squirt and a half-portion he had thought of her as a comely squirt and a half-portion with plenty of sex appeal.