Does anyone have any advice on creating a dual boot system, Linux onto a Windows 10 machine? I'm looking for such info as which Linux version, partition size on a 256G SSD (125 GB free), and other options I don't even know enough to ask about yet.

My aim here is to be able to crunch more effectively for this project (read: without the encumbrances of VirtualBox), and use it as a vehicle to learn Linux in general.

The way I do dual boot is simply to use two separate disk drives (SSD), and choose which one I want in the BIOS at boot time.
That keeps everything nice and separate. I don't want to know about boot loaders, whatever they are. They can only cause trouble.

As for Linux, that is easy. Use the latest Ubuntu, and get the LTS (Long-Term Support version), presently 18.04.4. The intermediate versions (e.g., 19.04 or 19.10) always have strange problems logging in, among other things. They just are not tested well enough, and you will spend more time on the support forums than anything else.

And when you have to choose which installation, don't use the default (first option). That does not provide a big enough root partition for BOINC (only about 20 GB as I recall).
Choose "Something else".

Things can get a bit tricky here, since you have to decide where you want to install BOINC, and make that directory big enough. Some people install BOINC directly in the "home" directory, which makes sense for simplifying permissions, since you are the owner of that directory and everything works (I think). In that case, you will want to make the home directory the big one, instead of the "root" directory. But I use the PPA version of BOINC (from LocutusOfBorg), which installs in the root directory. That makes installing and upgrading BOINC easy, but then you have to learn how to grant permissions and join groups, which is a big pain in Linux. You choose your poison.

For a 250 GB drive, I use the following, though you have wide latitude to change it:
• highlight each "free space" and click "+" and format with EXT4
• Primary partition; Mount Point "/" 200,000 MB (/dev/sda5) - EXT4
• Logical partition; Mount Point - "/boot" 512 MB (/dev/sda1) - EXT4
• Logical partition; Mount Point "home" - 5000 MB (/dev/sda6) - EXT4
• Logical partition; "EFI System" - 200 MB - FAT32
• Logical partition; Use as "swap area" - 2000 MB (/dev/sda7) - unformatted
• All the rest - unformatted

At least that works for me. I use Ubuntu only in dedicated crunching machines, and don't really have much in the home directory. I leave some empty space unformatted just for SSD over-provisioning purposes (to protect it), but you can use it if you want to. You will probably make enough mistakes that you have to do it over again often enough to make it easy eventually.