The 'Super Tiny USB Keyboard' from Brando is bigger than you mentioned, but still very small and low power consumption. Official dimensions are 170 x 73 x 15 mm, about 3x7 inches. I used one with my PDA for about a year before the usb connector got too worn to be reliable. I do not know if the keymappings would be compatible with the Raspberry Pi - I know that the symbols recognized by my Linux PDA were in different locations - I taped labels to the keys so I knew what symbol was where. And the antiskid strips on the bottom wore loose, so not the best contruction, but worth considering if nothing else in a small keyboard turns up that meets your specifications. I did enjoy using it until the connector wore out.
I want to give an exact URL but am on VNC and unable to copy/paste or to type certain essential symbols, so suggest the following in a search engine:
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