When you're downloading over TCP which checksums all transmitted data, there's very little chance of having a corrupt download with exactly the same size. If you're confident you're downloading from the official site (you normally are if you're using HTTPS and the certificate check passes), verifying that your download is complete is normally enough. Decent web browsers usually do the check for you anyway, saying something along the lines of "download failed" if they don't get the amount of data they expect, though I have seen browsers which just decide to keep the incomplete file without saying anything to the user, in which case you could check the file size manually.