Wallpaper size depends on its resolution and dimensions ... How about a website that automatically detects your screen resolution and recommends high-resolution wallpapers according to it?
The download time will depend on the theme size and your internet connection ... Read: Where are Wallpapers and Lock Screen images stored in Windows. You can change the lock screen image here ...
The option to change and remove your lock screen functions is a great way to make your iPhone fit your needs ... Open Settings. 2. Tap Wallpaper. 3. Tap Customize under your lock screen.