How do I revise an existing animated GIF?

I'm using Photoshop CC 19.1.3 & having a lot of trouble on this one. When I look online for an answer to my specific problem I come up with a lot of "creating an animated gif" from scratch tutorials, which generally don't address my specific issue.

I made a series of new frames to add to an existing animated gif; For starters, when I insert them into the existing psd they appear oversized even though they're the same dimensions as the original. (300x90 at 72 pix/inch). As a matter of fact (I just checked) it works both ways: when I copy a frame from the existing psd into the one containing the new frames, it also shows up oversized!

That "minor" issue aside, I have no clue how to insert them into the original psd's timeline without screwing it up completely: (When I tried I kept accidentally duplicating frames in the timeline, I think replacing existing frames with the duplicates.)

In other words...help!