how do you change the size of photo for emailing?

Do you have a Creative Cloud plan?  If yes and you wish to share the full size PSD file with a co-worker, upload it to your Cloud Space.   See link below for details on sharing and sending a link.

Share Adobe Creative Cloud files, libraries, and more

Note too that if you want to send a photo or other large file without losing quality there are many ways to do that, like DropBox or WeTransfer.

If you want transfering layered Photoshop files, you need to use a cloud service like dropbox or Wetransfer.

File->Export as gives you the possibility to save as Jpeg (among other formats), to resample and to check on the quality/file size. Save for Web is equaly interesting but not as flexible.

Use the preview to appreciate the visual quality of the picture, when modifying the Jpeg quality parameter. Look at the image at 100 or 200%, choose an appropriate image part and and lower the quality parameter as long as you get (in your appreciation) acceptable artifacts. Please note that if the source is a Jpeg, the file may contain artifacts from the previous compression. Artifacts will accumulate by each generation. Prefer the quality slider over resample. Only if the quality slider does not reduce the file size enough then use resample to make the image smaller. This rule is only true if you want sending a full resolution picture. If the aim is to send only a small preview then prefer resample.

You can resample the image (Image > Image Size) and save as a jpg or use File > Export > Save for Web and scale the image in that dialog.