First, I'd suggest you peruse this site:
http://www.krebsmicro.com/ Charlie Krebs is the BHS-DIC Photomicrography Guru. Pretty much all of your questions are answered there.
The projection eyepieces are designed to stay away from the outer edges of the field where image quality can be a bit less. The 2.5x is designed for full frame, and if you have the DI PA1-10A adapter, that's the correct one. The 1.67x does about the same, but the projected area is appropriate for an APS-C sensor. That's way too small for the full frame sensor in my opinion, although the SPlans are wide field so it may work out better than I imagine.
As PeteM suggested, a lower magnification objective would shrink the field a lot cheaper than the 1.67x. What he didn't mention is the addition of a BH-2 CA magnification changer such as this one:
https://www.ebay.com/itm/Olympus-BH-Ser ... Sw-1Zcgq5H
It also has the benefit of having a built-in Bertrand lens to help center your phase rings. You won't need a phase telescope.
I don't remember if there are vignetting problems using the mag changer along with the DIC intermediate tube. It's been awhile since I used mine that way. Mag changers are really nice to help with framing. I have a couple I use with my BHS-DIC system and BH-2 outreach scopes, and BX versions (U-CA) for my BX-50s.
I second the suggestion for an SPlan 20x if you don't already have one. They're very handy with pond life.
There are other ways to do it as well, but they will take some detective work on your part. No manufacturer I know of has a real 35mm full-frame sensor for video. Even Red uses Super 35. Matching field size to sensor size got a lot more difficult when the different aspect ratios and resolutions became available on the same sensor. Because of that, you are not using all of the sensor anyway, and the size varies, a lot, depending on the video resolution and aspect ratio you use. Each resolution/aspect ratio will have a different optimum projection set.
From Nikon for the Z6:
FX (36x24) image area: 6048 x 4024 (L: 24.3 million), 4528 x 3016 (M: 13.7 million), 3024 x 2016 (S: 6.1 million)
DX (24x16) image area: 3936 x 2624 (L: 10.3 million), 2944 x 1968 (M: 5.8 million), 1968 x 1312 (S: 2.6 million)
1:1 (24x24) image area: 4016 x 4016 (L: 16.1 million), 3008 x 3008 (M: 9.0 million), 2000 x 2000 (S: 4.0 million)
16:9 (36x20) image area: 6048 x 3400 (L: 20.6 million), 4528 x 2544 (M: 11.5 million), 3024 x 1696 (S: 5.1 million)
As an example, on my BHS I use a Panasonic GH4 camera and shoot 4K UHD. To get the best image I use a NFL 3.3x projection lens (to get any necessary eyepiece color correction) and a U-PMTVC 0.3x C-mount adapter to give me essentially direct projection (3.3 x 0.3 =0.99x), and fits very well. If I shoot instead 4K DCI, it doesn't fit as well.
Time to do a little research to figure out the best way to get what you need for what you are going to shoot.
Tom