Field of View
The sun is approximately 0.5° across in the sky. At 200mm on a DX (1.5 crop factor) body, the field of view (FoV) is 6.9° (wide) / 4.5° (high) / 8° (diag).
In terms of Sun diameters (☉), the 200mm FoV is 13.4 ☉ wide by 9 ☉ high.
According to Wikipedia, Mercury's diameter when viewed from the earth during May transits is 12 arcseconds. In terms of Mercury diameters (☿), your FoV is
FoV (☿) = FoV (degrees) * 60 (min/degree) * 60 (sec/min) / 12 (sec/☿)
This comes to 2060 ☿ wide by 1374 ☿ high. Using a Nikon D3300 (with 24.1 MP — 6000px by 4000px), Mercury would be approximately 3px across.
ND Filter for Photographing the Sun
I DO NOT RECOMMEND DOING THIS. I can't stress that enough. Certainly, it is possible to reduce the sunlight using an appropriate amount of ND filters to maybe, possibly safely take pictures of the sun without damaging your camera's sensor.
In my opinion, the only proper recommendation for photographing the sun (telephoto) is to reduce the strength of the incoming light to be safe to use at the widest aperture of your camera. That is to say, the aperture should not be relied upon to reduce the intensity of the light before it gets to the sensor. The reasons for this "rule" are:
To keep a simple safety rule simple. Analogous to the firearm-handling rule "the weapon is always loaded" (even if you think you know it's not), this rule is designed to keep your eyes and your camera sensor safe.
When photographing events such as transits and eclipses, the camera is most likely pointed at and focused on the sun for at least several minutes. The sun's intense light concentrated to some degree or another in multiple places in the lens. These places can become hotspots in the lens and damage glass elements, adhesives, etc., even before the aperture.
The correct, safest method to reduce the solar light intensity is to use a solar filter designed for photographic/telescopic use (see also Star Circle Academy's article on solar filters).
The sun requires about 16-17 stops of light reduction (around 1/1000 of 1% light transmission). If you have 17 stops of ND filters (this is darker than most welding glass. Only a welding shade number of 14 is enough), you'd probably be fine. However, I'm a strong proponent of "the right tool for the job", so I still do not recommend it.
Now, you mention you will be observing the transit around 6:30pm. Depending on your longitude, the sun will be 15°-20° or so above the horizon. This will help reduce the amount of light reduction by a couple stops at most.
Having said the above, you will find examples on this site and others of great pictures of the sun with a Venus transit or sunspots, taken with something like a 10 stop ND at ƒ/22 or so. I think every one of these pictures should have a "Don't try this at home" caption.
Oh, and your UV filter would contribute to the safe ability to shoot the sun about as much as mosquito attacking a fighter jet.