re: What is the correct image dimensions for Slim Plus entries so they aren’t cropped/resized into oblivion after being uploaded?
The image a scaled and cropped to best fit the dimensions of 100x120px. See this FAQ for more details on sizing images for Slim Plus. The relevant part of the FAQ is in the first “Exception”
re: is there a way to list all entries rather than broken into multiple pages? I’d rather have one long page then multiple short pages.
Yes, the page limit is configurable and can be disabled. See this doc page. One word of caution … I do not recommend showing to many at once. Page load times will suffer for the same reasons WordPress defaults to showing 10 or is 5 blog posts at a time. I don’t think I’d go over 50.
Hope that helps!