Open the size menu first, not the photos. A menu with chest, length and sleeve figures in centimetres is worth more than ten styled photos. Then look at the stock beside each size: a zero means that size cannot be bought, whatever the cover suggests.
Next, check whether colours have names or numbers. Numbers are fine if you write the number and the colour in your order remark. Last, check the price per option: some listings charge more for certain colourways, as slip 7 does.
Lolobuy's size guide includes reference charts for men's and women's tops, like the one shown here; compare its chest figures with a top you already own. The fit notes page walks through measuring.