I am at the lower range of height at 5'6" and ride a 51cm conventional frame.

But in Giant- I fit an XS-an S and just onto M. According to the Giant Geometry chart- I should be top end of the S sizing and just into M. I have the feeling that Giant have the chart a bit on the small side. The XS is just a bit small in standard form- but the seat tube adjusted to height and I fitted a 100mm stem instead of the 80mm supplied. The S is fine but might shorten the stem.

Only way to check is to test ride but as 6'3" is just at the top of Giants quoted L sizing- I think it will fit. Just make certain you keep the seat tube within its limits

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