I have a few trusted online shops (be/nl located) where I sort of know sizes and style, don't like 'real' shopping much.
But for sizes, quite important you have +- proper measurements so better not if urgent. Depends how well you fit in standard sizes. (example... I know my shoulders are too wide so have to keep that in mind but won't matter for a pair of pants). (not in the habit of just ordering 50 different sizes and keep only what I really want)
Be careful using size tags of stuff you are wearing but does give a baseline. (certainly if you shop in same shop and same type of clothing (pants & jeans are often different sizes), it might help to have a note with them). Check stuff you like wearing.
Knowing some brands that usually suit you make it faster too. If you wear something you like, note brand, fabric, size... Not entirely to get the same thing again but you might find something similar (in my case fabric important).