How big is a Strava tile?

Short answer

    The tiles are about a mile square.

A longer answer:

The most zoomed in tiles that are used on this site (and are recorded on VeloViewer) are the "level 14" tiles from the OpenStreetMap. To get to level 14, you start at level 0 which is one single tile representing all the world below about 85 degrees North and above 85 degrees South. You then zoom in to split that tile into 4 equal-sized tiles, each with a side half the length of the first tile. This gets you level 1. Then you zoom in again splitting those four tiles into four tiles each, making a total of 16 tiles covering the whole world at level 2, each with a length of a quarter of the length of the original. So at level 2 you have a total of 4*4 = 16 tiles.

Do this splitting procedure 12 more times and you get to level 14. So there are 214*214 tiles covering (most of) the Earth. That is 16,384*16,384=268,435,456 tiles! To determine the size of the tiles, we first need to think about the "width" (i.e. west-east distance). The width must vary depending what latitude you are at because there are 16,384 tiles that you must "fit in" no matter the distance around the world at the latitude. I.e. as you move away from the equator the tiles get smaller. At the equator, the width of each title is S = (circumference of the earth)/214.

To get the formula for the width of the tile we need to think back to our teenage trigonometry lessons. I live at 51.5 degrees North so, for me, the formula is

Size = Equatorial circumference of Earth * cos(latitude) / 2^14
     = 40,075km * cos(51.5 degrees) / 16,384
     = 1.523km
     ~= 0.95 miles.
For the height, we don't actually have to have the same constraint that the tiles must "shrink" as we move from the equator, but according to the Web Mercator convention used by all the main mapping websites (Google, Apple, Bing and OpenStreetMaps), the heights are also scaled at each latitude so that the tiles are (very close to) square. Here are some tile sizes at various latitudes
LatitudeTile size
0 (equator)2.45km*2.45km
30.0 (e.g Cairo)2.12km*2.12km
51.5 (e.g London)1.52km*1.52km
71.1 (e.g Nordkapp)0.79km*0.79km
85.05 (limit of tiles)0.21km*0.21km

So there you have it, if you want to hover up some tiles really quick, head north. Just don't blame me if you are chased by a polar bear!

OpenStreetMap has a page all about zoom levels.