There seem to be two heuristics for determining frame size without testing a bike: inner leg length and body height. I have an inner leg length of 85cm and a body height of 192. According to my inner leg length, I should buy a 55cm frame. According to my body height a >60cm frame. Any advice on what to do there besides testing? Intuitively, I would think that I should buy the highest frame for which I can still put the saddle in a correct position?
If you have proportionately short legs and a long body and arms, I would recommend against buying a bike unseen. You really need to test ride some bikes and get an idea of the geometry you need.
You’ll need is bike that has a sufficiently long ‘reach’ (horizontal distance between bottom bracket and bars) so that your upper body is not cramped, but a short enough seat tube so you can set a shorter bottom bracket to seat distance. You probably also want a short ‘stack’ (vertical distance between bottom bracket and bars) so the bars are not too high.
You’ll need to seek out and try a number of bikes to find a model that suits you.
A point of note here is that cyclists differ in whether they want or need standover clearance (the term for having room between the top tube of the bike and your crotch, the lack of which can be awkward or injurious if you find yourself suddenly coming forward off the saddle for any reason, such as a panic stop or fumbling getting over a technical bit). Some people's riding habits and skill levels make having little or no standover acceptable and for others it's a huge problem.
There are at this point some highly divergent schools of thought surrounding the need for standover in good bike fit. The retail industry has largely adopted a dogma of everyone needing at least some, because they have more of a stake in protecting people from themselves, not getting sued, etc. Meanwhile, many real-world cyclicsts who ride traditional level top tube bikes find that by the time they're on a frame size that feels right for all the other reasons, they really don't have any standover. And so there's a lot of contradictory advice and rules of thumb out there surrounding the topic.
In your case, it's important to understand that you won't get any standover clearance with level top-tube (aka non-compact) bikes that are more in line with your overall size. You'll primarily only get it with compact geometry bikes, or in other words bikes where the top tube has a lot of downward slope.
Generally speaking, to avoid feeling cramped due to their long torsos, people with your body type often need to look at getting a longer stem even with a frame size that's around par for their overall height. If you went down to a 55, a long enough stem may not even exist, and ultra-long stems can also do screwy things to the steering geometry.
If you really must buy without test riding, go for the larger size, and if you're not confident you don't need standover clearance, get a bike with a lot of top tube slope.