I carry a laser distance meter for that sort of things. Also wall/hedge/fence heights, etc. Besides, you can just ... not answer certain quests if you're unsure of the answer (or need to use estimation – there's tagging for estimated widths, but it's not used by StreetComplete). But I also found it handy to figure out the sizes of common paving stones, which makes sidewalks rather easy to measure without actually measuring :)
