Gallery

Once a gallery is created can I go back in and edit the pixel size that was original set for that gallery? The gallery already has photos in it with all captions so am hoping I do not have to make whole new gallery and add all captions again.

You can upload any size that you want regardless of the settings. The setting only causes a warning when you upload an image that does not meet the requirements that you determined when you created the gallery.
If you want the images a different size, you can upload images of a different size.
Changing the settings for the pixel size of the gallery will have no effect on the images already uploaded.