newbie: how to get larger keys on lockscreen keyboard?
Considering that you responded in the comments that landscape-only is alright, I'd suggest Ultimate Rotation Control. Use its "Forced Auto" mode to rotate any apps to any direction you want - that of course includes the lock screen. After proper settings, it could seamlessly integrate with the built-in rotation switch too (basically you could toggle the built-in switch and the lockscreen will now obey).

On my devices, when I have a password set and lock screen rotated to landscape, tapping on the password textbox will show a full-screen editing interface with landscape keyboard, which should be good enough for your fingers.