Welcome to the forum! Like you, I'm 5' 9" and I've always been more comfortable on a horse that fills out my legs. My previous horse was about 16.3 and that was great. My current horse is a draft cross and he is currently 17.3 and growing and I love him to death. He has a great, calm mind, loving personality and like the gelding you are looking at, he is extremely comfortable to ride.
Just a few things that are less favorable about him so if you buy this gelding you go in with your eyes open.
A horse that big eats ALOT! My guy tucks into 6-7 flakes of grass hay a day. He doesn't need much in the way of concentrates (pellets).
I have had to search to find tack that fits him appropriately which is doable but you most often can't just walk into any tack store and find a bridle, saddle, girth, blanket, bell boots, halter, etc. that will fit your horse. I've had to measure my colt and order most things on line.
I am currently in the market for a new horse trailer because he really doesn't fit in my standard 7' tall X 6.5' trailer anymore.
I have never had this issue but I guess a lot of farriers either refuse to work on the big guys or charge you a lot more.
When worming him, I have to use more than one wormer to get the correct dosage.
If you are boarding him, the standard 12X12 stall would not work, you'd need at least a 12X 16 which could be more expensive. I keep my guy out in the pasture.
If you can afford the extra costs involved, then none of these things are reasons not to buy him.
Find out why your trainer feels he is "too much" horse for you and if she can't come up with a good reason, follow your heart