Well you are not going to find any email app that's less than a MB in size, and I don't really believe your Gmail app is either. Perhaps your settings are misestimating the size because the app is on the /system partition (i.e. part of the ROM) rather than in /data (where user apps live). On my tablet the installed GMail app is 18MB, while the download size for GMail in the Play Store is 9MB (not 0.9MB).
That's the other thing: the size in the Play Store is the download size, and the installed app will be bigger (by how much may depend on your Android version). For example, Aquamail on my phone is 21MB (app only).
So if you are that worried about size can you not set up a second account using the existing GMail app? Or does the phone not have another built in mail app (many do have a manufacturer's app in addition to GMail)? That would add the least to your storage.
If that doesn't work, the veritable K9 Mail app has a download size of 3MB, so it's likely that the installed app is also smaller (it's been several years since I last used K9, but while it isn't pretty it is functional and reliable).