Somewhat related to this, including a scale object in the observation photo is often recommended or suggested. This might be a pen or coin, ie a common object that others will have access to and can measure directly themselves. I have seen a lens cap used, which is variable in terms of lens diameter, but not hugely so. For large things, like trees, having someone stand alongside is a good indicator of size, but again quite variable!
I gave a bit of thought to this, and it occurs to me that a pen, for instance, could be at a slight angle to the camera, and thus appear shorter than it really is. This would have the effect of making the subject appear larger than it is. Still considering the pen, if you angle it sideways (ie rotating in line with the barrel) then it doesn’t distort. This is true of any circular object, that no matter which way it is tipped, the apparent longest diameter will still be true. So it follows that the best “scale objects” are those that are circular, as well as readily available or familiar to others. Coins of course are country specific, and in the age of plastic are less likely to be at hand when needed, but are able to be sized even if you don’t have them handy, as you can google and find information on the coins of most countries!