What determines the resolution of a parameter, i.e. 0 - 65535 vs. 0 -255

Some of my fixtures parameter resolutions are different than others. For example, the pan and tilt on my Intimidators go from 0 - 65535 while others are 0-255 (or percentages if set that way.)

This is also true for the Intensity channel of my Colour Profile 5’s.

Is this a preference, or is this a fixture definition?

This is based on what fixture personality You choose. Some are 16 BIT, some are 8 BIT

Personality? Are you referring to the fixture definitions built in to Onyx? Is there a way to edit these preferences through XML or something of that nature?

You can edit the fixtures via Fixture Editor, but here it’s not the case.
This depends on Your fixtures channel mode as well.
If Your fixture is having 16Bit Pan/Tilt You want it to be 16 bit.
If the encoder resolution is too slow in the 16 bit mode, You can switch the Encoder sensitivity between 8 and more, up to 16 bit if that will help You.

This option You will find described here:

Ahh, yes, well my “encoder” is my mouse, which isn’t ideal, naturally.

Try to use the mouse wheel, gives more accuracy.