The thread TT unearthed explains several things about the available size options, but let me clarify a couple of things:
a) Thumbnail, Medium and Large are supposed to be three preset options for downsizing larger images, so the Large option isn't available when you upload an image that's smaller than the default for that option.
b) The Full Size option means original size, if the width of the original is smaller or equal to the maximum the theme allows, or downsized to that maximum, if the original is larger.
Now, re the 1024x1024 you mentioned: I suppose you saw that in Settings > Media. Well, it means nothing. As I explain in the thread TT linked to, the media settings are there for you to set in accordance with the theme you're using. You're using Chateau with no sidebar; maximum in this case is 791px wide, so you can't "retain" the size of a 1000px wide image. If you upload a 1000px wide image, it will be downsized to 791px if you select the Full Size option: if you select one of the other options, it will be downsized to whatever (reasonable) width you've set in Settings > Media for that other option.