what you need first is your baseline measurement.

that means understanding what bike fits you. size frame, stem, bars, crankarm, seat height, and take that geometry down

then you can work your way out from there

do you want endurance road bike geometry that presents a shorter reach, lets you sit a bit more upright, neck tilt isnt so severe

do you want racebike geometry that lengthens you out, lets you get more aero? do you have the strength and flexibility for that?

now start shopping bike geometry charts

every brand has their own flavor of what they do for geometry. ive shopped these charts for awhile and Bianchi for instance is wild, i sometimes wonder if italians were born with short arms and wingspans

saying buying a smaller frame isnt the complete picture, you have to understand where hpyou want your feet, butt, hands, to be

then its a matter of finding the correct mix of frame brand and parts to get there

i shopped these geo charts for 2 years while saving up for my next bike, using my baseline. found a brand that worked, and now i have more than one new bike from that same brand because i know what geometry it delivers

and i actually run two different sizes because depending on the bike's mission, i know how i want my body positioned for it. when people look at bike size, they always just look at the frame. i dont. i look at the persons butt, feet and hands

to help you find your baseline, if your body is proportional, a 90mm stem is ideal for an agressive race bike fit

if you have longer torso or arms, shorter etc.. this is where you really start to take advantage of those short or long stems. some people can use up to 130 140 size stems

it takes awhile to understand your body's needs, so buy a small tape measure, and start experimenting wit the bike you currently own