I was recently offered a one day per week share similar to this. Pure bred shire, 18hh and 5 years old; I am 5ft! He was the biggest horse I have ever ridden and incredibly comfortable, he had a sitting trot to die for - like sitting in an armchair as a couple of people above have said. I did feel incredibly safe on him but turned him down because of:
1. I would have been largely on my own with him as opposed to a yard with lots of help if you need it. I simply could not even tack him up on my own. With such a huge boy I was reluctant to stand on a stool to tack up just in case his big bum knocked me over accidentally. Too risky! I felt I could not properly care for him to the extent he would need due to my lack of height, which I felt was unfair on him.
2. Even though he felt as safe as safe can be, he had (as young horses often do) some things that needed working on. In the school he napped and when he did it was a huge effort to actually move him on. I worried that if out on a hack and he napped that this could potentially be dangerous, depending where he decided to nap, as he was not as easy to move on as a smaller horse. I felt he needed taking back and going over this with someone more capable of doing so with him. He also planted his feet when not wanting to come out of the field, and again I could not physically move him due to his size. Again something I felt I may not have been able to work on with him. He also had a bad habit of head throwing with a tighter rein (I have soft hands so he didn't do this much) but when he did throw his head as I wanted to see just how much he would do this, for example when asking for a sudden halt when out on a hack, he easily looked and felt more than his 18hh with the height he threw it, which for me was not comfortable.
Its personal preference as to how you feel on him, but I would also bear in mind things like if you might need a hand putting a rug on him, are there people around who could help? That's a huge factor as to what put me off. A shame as he was gorgeous and a real gentle giant. A few hands smaller however I have found much more manageable for my height; my share is 16.2 and chunky and this feels about right. I do also ride a 13.3 though and love him just as much. One of the perks of being so little I suppose is that we can also ride the little ones!
The only way you will know is to try, the age is the thing more likely to overhorse anyone, a 5yo is very different to a pushey/strong/not perfectly behaved 8/10/12/15 year old, they can just be more unpredictable or not understand which can make riding them more of a challenge.
