Hmmmmmmmmmmm ... solutions before knowing the exact problem. 

First we need to have more information ...

In Photoshop (as shown in the attched sample), you can specify both the number of pixels and the document size.  You can also specify "Resolution" in pixels per inch (PPI) ... that figure may get used when the image is printed. 

After you have done all of your image editing you should save the Photoshop Document (.PSD) with all of the layers ...,

Then you might want to also create a JPG for uploading and/or printing.  You change the pixel count with Image -> Image Size.  You might want to reduce the number of pixels in order to reduce the file size for uploading, etc.  You also might want to set the print dimensions and/or reduce the PPI resolution

BTW you should NEVER uncheck  "Constrain Proportions" unless you intend to distort the shape of the imgage. 

If you have a photo quaity printer, you might want to use print preview: File -> Print with Preview, there you can check "Scale to Fit Media".  This applies ONLY to your own printer -- not to what happens at a photofinishing shop. 

Please ask more questions if I have led you away from the light (of understanding). 

--Martin