I have used the Google Maps app on Android with Offline-maps a little.
Works fine. I have chosen a region to download and it wouldn't do it because it was "too big". In the particular case I chose a smaller region and all was good. No idea what limits are.
Also beware of maps timing out and getting automatically deleted. Timeout date is marked if you look for it. They simply die without notice. The phone doesn't understand "but I don't care that it might be a little out of date" shouted very, very loudly. No maps !!!!!
Just to add to earlier comments, I know someone who uses off-line UK OS maps on his phone for walking. It's well integrated with GPS. Shows position, tracks route if required. Might need external battery for extensive use.