My giants are twenty feet tall.
Half-giants are half that, around eight to ten feet, and so are most other lesser giant variations (Fir Bolg, newer generations of Fomorian, Ettin, etc).
And spriggans count as both giants and fay. They are supposedly either the distant relatives of giants, or the ghosts of giants, and were guardians of fairy courts, grave sites and ruins. Powerful. Their kids would likewise grow to about eight to ten feet until they reach maturity.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)