The thing nobody tells you about brown sandals is how the colour behaves once the sun has had a go at your feet. A tan line creeps in around the straps within a fortnight of regular wear, and suddenly the gap between the leather and your skin looks less deliberate than it did in April. It's not a flaw exactly, just something worth knowing before you plan an outfit around a flawless ankle.
Brown doesn't sit well with black. I know that's an old rule plenty of people have happily broken, but on sandals specifically, where the leather is doing most of the talking, black trousers or a black bag next to warm brown straps can look like two decisions made on different days. Navy is friendlier, and so is anything with a hint of red or rust running through it already. Where brown sandals properly let you down is on very formal days, the kind that want a sharper, cooler shoe, and on anything involving a lot of standing on hot pavement, because most leather straps need a short settling in period and will rub before they behave. For sizing, go true to your usual fit and pay attention to where the main strap sits across the top of your foot rather than the length of the sandal itself. Too high and it digs in by midafternoon, too low and your foot slides forward with every step, which is how blisters start. If you've got a narrower heel, look for an ankle strap with some adjustment in it, because a fixed one that's slightly too loose is the most common reason a perfectly nice sandal ends up at the back of the shoe rack after one wear. On finish, the easiest colours to live with are the mid tans and the slightly aged, worn looking browns rather than anything glossy or very dark chocolate. The lighter, matte ones hide scuffs and watermarks far better, and they don't scream for a matching bag the way a shinier finish tends to. A deep oxblood or chestnut is lovely in photographs but shows every scrape by August.
Put a pair on with a plain linen dress you already reach for every summer and they'll quietly make it look like you dressed on purpose, not like you grabbed the first thing by the door.