When I save my file my image is reduced

Resolution is Printer pixel print size you cans print your images pixels any size just set the ppi you want..  You need a lot of high quality pixels for a  high quality printed image.  You state the PNG image for the web size is good resolution is meaningless on the web displays have a fixed ppi. Image are displayed  with the display's ppi.  All the matters on the web is the number of pixels. Its quite normals to reduce the number of pixels your  have for an image when saving PNG files for the web. Print size images have more pixels then a displays can display.   So you resize the image size for the web to have fewer pixels and smaller file size. The small png image will transfer across the web faster then a print size image and the smaller png image will fit on a display. 

A 4k Display displays a 8.2MP image. Most displays have fewer pixels.  A 1080P display displays a 2MP Image.  Digital Camera image are too large for displays.  Image  for the Web are resized to fit on a  display. The image quality is lower then the Print size image there is fewer pixels to store details in.

You say that the PNG size is good, but what kind of format do you have problems with? Jpg? PSD?

What are the pixel dimensions of the image before you save it, and what are the pixel dimensions afterwards?

Please post a screenshot of the entire Photoshop window with the image open just before you save.

Make sure that the Layers and History panels are visible, and that the status bar is set to show Document dimensions.

Do not attach the screenshot, use the Insert Photos button in the toolbar.