NavList:

You might be able to see it by projection with that "monocular". You will need excellent focus, and you may want to hold a clean white piece of paper as a projection screen and move that around in a steady circular motion to see details without interference from the fine grain of the paper.

Mercury is quite small. It's angular diameter in transit will be something like 0.2 minutes of arc or about 0.7% of the Sun's apparent diameter. The last transit of Mercury was on May 6, 2016. I observed it by eyepiece projection with 60x magnification from a 6-inch reflecting telescope. 
NavList discussions of that transit: http://fer3.com/arc/sort2.aspx?subj=mercury&author=&y=201605&y2=201605

Note: transits of Mercury are dull. At most there is some fun getting it all set up and saying "Hey! There it is. It works!" But then all you have is a little black dot like a speck of pepper. And it moves quite slowly. The events of ingress and egress have some small excitement to them. Small excitement. There should be breaks in the clouds during the morning here in Rhode Island so I may try to watch for a few minutes. It's expected to be a warm day before a serious cold snap, so I should probably get outside!

Frank Reed