Am 5ft 8 and am an advocate of Youth or Small Men's size batting pads, depending on brand.
I have tried Salix and M&H Alpha in Youth ; Kook and G-N in Small Men's. On balance, the latter size are a better fit as slightly longer and knee is positioned perfectly. Not much in it and if Youth's are priced lower still worth considering. Will vary per brand and yes instep to knee point is key.
(in gloves Salix Youth, M&H Youth, Kook Small Men's all good; standard Men's in G-N were good too, a pair of Legend Small Men's felt restrictive but may have loosened up. Kook Youth too small).