Motor spun free 15 years ago. Sitting under roof since then. Filled cylinders w/Marvel Mystery Oil a month ago....still won't move. Not sure how big of a cheater bar to put on the crank bolt ??

Try turning the crank clockwise, then Counter-clockwise back and forth, again and again. If you start to get some movement and then it improves each time, you will probably get it to turn over.

Back and forth is your friend. Do not keep trying to go just one direction.

You say it was under roof which in my thinking it was exposed to the weather. Was it ever spun over during that time to keep oil all over the parts. If not you should pull your valve covers and see if the rockers have surface rust.

I was trying to get a 400 Pontiac to turn over a little while back that has been in a barn not started since the mid 80's with no luck. It's rockers are covered with rust as are the cylinder walls so I'm thinking the whole interior is in that condition.

Used both Seafoam and Marvel.
By the way I was using a 4 foot cheater on the breaker bar and loosened the crank bolt without as much as budge of the crank.

If you can get to the back of the engine,take a 3 ft piece of bar stock,drill and bolt to crank flange,you'll do far better that on the bolt.

Another tick is to fill cylinders with diesel fuel and let it sit overnight....

Just throwing ideas............

My solution has always been a combination of diesel, ATF and WD40. It has never failed me yet. Even unstuck a 318 that sat with no carb for 10+ year. Engine almost ran afterwards too!

SILLY QUESTION
if a motor is stuck that bad, what does one expect to achieve by "breaking it loose" Another 100K miles ??? Not likely

get it running
then what ?/ figure out it needs rebuilding anyway

It is in a 442 convertible I have for sale that needs total resto.
I would at least like to assure any potential buyers the motor doesn't have a rod through the center of the block