Recommend me!

Stop at the Red Light http://www.mangago.me/read-manga/stop_at_the_red_light/ has a big height gap.

Sweet Heart Trigger: http://www.mangago.me/read-manga/sweet_heart_trigger/

Any of the SNK dj's (Attack on Titan, Shingeki no Kyojin) will give you a height diff.

Article 69 of the Criminal Code's manga has a big seme and a tiny uke. some here, many on MRM.
DEADLOCK tall seme, 5'9" uke. and another big big seme (we don't know if he is going to be with uke or not at this point) http://www.mangago.me/read-manga/deadlock/

End of the Road (What Lies at the End of the Road)

http://www.mangago.me/tag/small%20uke/
here is a list from doing a search on "small uke"

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