scared to upgrade to el capitain/photos app & losing my iphoto library housed on external hd

How will photos read my iphoto library if its housed on an external hd? has anyone had success with this?

Sarah, it will depend on the file system of your external drive. Photos can only migrate your iPhoto Library to the new format, if the external drive is locally mounted and correctly formatted. The filesystem of the drive needs to be MacOS Extended (Journaled).

So, check the file system of the drive. Elect the drive in the Finder and press the key combination ⌘I to bring up the Info panel:

If the file system is different, you will need to move the library to a correctly formatted drive.

No. Photos simply migrates the Library to its own format. The iPhoto Library is left untouched.

How large is this library?

As TD said, your iPhoto Library will be left untouched.

This document describes, how your iPhoto Library will look in Photos: How Photos handles content and metadata from iPhoto and Aperture - Apple Support

If you have unfinished projects - books, calendars, you may want to finish them in iPhoto before you upgrade. The templates for books have changed.

If your iPhoto version is listed in your Purchase History at the App Store, you may be able to continue to use iPhoto after the upgrade to El Capitan by updating iPhoto to version 9.6.1, so you can finish any projects or compare the migrated libraries. But in the long unit will be better to migrate the library to Photos.

The disk space used by the Library is not significant and no, that's not an especially large library. I have one twice that size.

To migrate when the Library is on an external: Hold down the option (or alt) key key and launch Photos. From the resulting menu select 'Choose Library'