Help saving 47GB psb file into usable image

It's not something I have done for a very long time, and I am wondering about the workflow you could use to downsize each file before merging. Even at 300DPI a 36 inch print would be 108,000 pixels wide, but I can't imagine 300DPI making sense for the sort of distance a print that size would be viewed from.   200, or even 150DPI, would probably be OK,  If you are determined not to compromise, then there would be zero advantage of aiming for more that 108,000 pixels wide. 

Dag, (D Fosse) knows a lot more than me about printing, so I am sure he will be able to advise you.

@Bldeagle1 You can use Image > Duplicate and create a new merged file, where all layers are combined into a single layer. This will reduce overhead. Close the original layered file to free up memory. Then resize as @D Fosse wrote and save.

I can guarantee that you will never need 250 000 pixels wide, ever. 

Open image size, check resample, check the link to constrain proportions, and set a sensible pixel size. 

How much is up to you, but if you make it about 8000 pixels high (that's the short side), you still have plenty for any reasonable purpose.

The physical print size is totally irrespective of this. You can print any file at any size as long as you have enough pixels for the purpose. That is not a problem here 🙂