I’m 5ft 8 and have ridden a small an medium plug, though medium felt a bit tall for me and required a 70-80mm stem, while I could use a 100 with the small. Don’t know if current geo is same. 17 MTB is small for 5ft 11 I think, but just at the limit with that inseam I suppose?
Find out the effective top tube length and compare it with the Giant, and also the seat tube angle if possible – steeper can make smaller sizes it feel much closer to handlebars than a simple 1cm difference in effective top tube length. Though slacker can feel less scary going downhill, if between sizes it can make the reach to bars seem a stretch.
The top tube on my older plug is short for the given size, so you’d probably be safe with large, but get the numbers.