So here's something I have not seen many guides on on the internet in general: how to get a rough idea about the size of a used bicycle from pictures.

It's not perfect and if someone can just tell you, good! But honestly, most people can't or won't tell you the actual size of a bicycle (that is, framesize) in any useful way if you're buying online.

Especially during times of covid, but just in general, you probably want to avoid making a trip out to some promising bicycle only to get there and realize that to you, it's either a monstertruck or a clown car.

General Sizing

I'm not going to get into the nitty-gritty of it all, it's sort of an art of itself. What you want to do if you're not well versed in the subject is look up a brands recommendation for the type of bicycle you want to buy.

Say you want to get a roadbike, look up what size roadbike different manufacturers recommend for your height to get an idea on what size frame can theoretically fit you. It'll always come down to like personal measurements but it gives you a starting point.

Oftentimes, people will list the wheel size (most probably 26" or 28"). Unless you're really small or really tall, this matters less for sizing and more for how the ride is. You want to look for frame size.

You can also often get people to give you a wheel size. It's a lot easier to measure than frame size. If you have no clue, for an adult bicycle assume 28" Wheels, 26" have largely fallen out of favor in the western world.

This guide is no bueno for actual mountain bikes. That is, the type you can huck yourself off a cliff with and somehow come out fine with enough skill. Those are expensive as hell though and if you're a beginner, you probably do not want to spend large amounts of money on a used bicycle in general and especially not if you want to huck it off cliffs and make sure to come out with all your teeth in place.

"Mountain Bikes" (i.e. in the style of) this will fit fine. They can be good bikes still, I'm not trying to drag them, but most mountain bikes you see you do not want to huck yourself off cliffs with. They're town or light offroad bikes and people buy them because they're cool, which is fine.

How to figure out sizing on step-over frames

These are also called mens bikes but obviously that's a stupid name. They're the ones where to get on them, you have to swing your leg over the saddle and have a top tube running from the seat to where your bars are.

These ones are easy: you want to look at how much space there is between the top tube (between seat and bars) and the bottom tube (between pedals and bars)

The larger this space is, the bigger size the bike is. Compare some fotos on different sized bicycles for the type you want to get and you'll get a feel how much space there should be for you. Again, not perfect, but if you're 6"2 and see a bike where the down tube touches the top tube with no space, it ain't for you and vice versa.

How to figure out sizing on step-through frames

Or womens bikes, again, stupid name. They lack the "meeting point" as described above, so this gets a bit harder.

You generally want to look at how big the part between the wheels is compared to the wheels themselves. Unless you're looking at a rather funky bicycle, there just isn't a way to make the fitting for the rear or front wheels larger because...well the wheel size don't change. Make the seat- and chainstays larger (the one going from your saddle to the rear wheel and pedals to the rear wheel, respectively) and you just have an oddly long bike. Cargobikes sometimes to this, but unless it's a cargo bike, that distance is pretty much fixed.

So you know, look at the tubes between the wheels. Are they about as tall as a wheel? Smaller? Bigger? This is obviously a lot harder to estimate, break out a digital ruler or something if you must. Beats getting out there getting disappointed.

Another good indication is how high does either the part where the frame meets the fork goes or how high the bars goes. Step through frames have an upright sitting position by design (unless you do some odd things to them) and the higher that goes, the higher the current owner is or the frame can support without making it a nuisance to ride.

For both of those: compare to wheel size. Is it like 0,8 the wheel size or more 1,2 the wheel size and so on. Then compare to brands selling you them as to how it shoud look.