Help needed with cosplay prop build!

What kind of advice are you looking for? You can't paste in images here. Only links.

My apologies for such a late response! I've just now added the necessary links.

I need any advice I can get on how to properly set the model's dimensions to inches. There seem to be error messages when trying to work with this model in inches likely because it's too big. I assume I'll have to further segment the blade halves into more printable sized pieces and should also probably add notches of some kind to reinforce the halves when assembled.

Why would you need to set the model's diameter to inches? The 'save-able' work space in Tinkercad is 1000mm in X-Y-Z. Changing it to inches does not increase the work area. If your project is larger than that, you should either work in a smaller scale, subdivide those parts, then enlarge them to the proper scale. You can ad 'notches' but I like to add holes for inserting metal pins (or coat hanger wire). This will aid in ensuring the parts are aligned when gluing and will also add a bit of strength. If there is a metal rod that reinforces the sword from end to end, this would be the strongest way.

I'm unsure of what the cost will be to have it printed. That can vary greatly depending on who you contract out to do it. The material you choose, size and time it takes to print, play a critical part.

You can print a lifesize Boeing 747 stl, and long as the model is segmented to fit the print bed. If you're using a printing service, more than likely they would have the printer that could handle the width and the thickness without issue, but the length would be a problem. I don't think you will have an issue if the parts are segmented to 10 inch lengths. Orientation of the print is also critical to it's strength. If printing in PLA horizontally printing the sections would give you better strength, but the downside will be that the layering (staircasing) would be evident. Printing the sections vertically, would lessen the layering, but wouldn't give the best strength. Generally the bonding between layers in PLA is the weakest point.