An old thread but I have an solution as this is a problem that still exists.
I have just released an AppleScript app that will automatically resize a selection of photos from Apple Photos, provide you with three different size options and then attach the selected size option to your default email client.
The script is only compatible with Postbox and Microsoft Outlook, but it saves a lot of messing around when trying to send bulk photos and deciding what scaling to apply to keep your email size down.
It is important to note this script adds the photos as an attachment not as an inline image.
There is a little user interaction required when first installing and running the script, so please read the install instructions in the Readme doc.
It can be downloaded from https://github.com/tnickels/ePhotos