This is a big issue among paso fino owners and breeders so first the 50 cent explanation
If it staggers when you get on and has huge sink in the fetlocks when you walk or gait, you're too big.
Now the 5 dolllar explanation
Height is one of the absolute worst traits to use to determine if the rider is oversize. And weight isn't a lot better because is you use the 25% rule on a fat horse it will be overweighted and if you use it on a real tank that's strong as an ox and all you do is corto around the arena twice and get off the horse can actually carry a bit more if the ground is firm and it has great suspensories also and is fit and muscular.
If you seriously have any doubts, video yourself riding the horse. But do remember just because your butt may be bigger than the horse's butt is no reason to get a larger horse. You might want a bit longer saddle though if you have a lot of rear overhang, and maybe a sport bra.
In the US we have this general rules that the bigger it is, the better it is. Baloney. I and my SO ride 13 2 paso fino mares. He's 6 1 but slim. I'm 5'5 and medium (from 125 to 140 depending on which year you're talking about) These mares have no problems at all carrying us and we've logged hundreds of miles on them. His favorite is good for about 8 miles. Flexy suspensories and lightly muscled. Mine has done 30 with no problems. At hunter pace rides they all think she's a Conemarra pony - a real tank.
If you watch old cowboy movies and look at art from the middle ages you'll see that people who rode horses all day long for long distances did NOT ride tall horses. They rode STURDY horses that stayed sound, were tough, healthy, strong and willing (and gaited). These are people who had to get on and off 20 times a day maybe. No mounting blocks or trees stumps were available. They didn't WANT tall. I know several rodeo cowboys who are always selling their horses because they're too tall.
When we go on trail rides you can guess who gets off to move limbs or open gates - us on our short horses. We ride for three hours, then go dancing while others are wrapping and icing sore muscles and eating pain killers. You can get a serious strain just getting on these monsters. I know because I have a bunch of them next door and I'm " ARE you CRAZY? (when I'm asked to ride one.) If I fell off that thing I'd be killed. It's as high as house. My mama didn't raise a fool!"
If I want big I'll buy an elephant or at least a camel. I LOVE short horses!!!
By the way - don't trust a weight tape. They are really off a lot. Our 14 hand endurance paso fino is 925 pound. Her 14 1 sister is a bit more but can't carry as much - weaker front suspensories. Our 13 2's are 750 or so when they're slim and fit. Probably 800 now. We have at least 5 that are over 1000 pounds at 14 2 or a bit more. WE just really prefer the short ones.