While I think 4096 RSA is strong enough, and would never been the weak link, Chromebooks supposedly have have embedded 8192-bit RSA public key. The fact that gpg does not enable 8192 by default is probably a damn good indication that larger keys have no extra benefits. Nevertheless, in the event that the Chromebook makers know something that we do not, or you want to make an unreasonably large key because you're bored,  you can make one by replacing NAME, COMMENT, EMAIL, and PASSWORD with the relevant info and pasting it in the terminal on Ubuntu. It will probably take some type to generate that much entropy, so move your mouse around, type random things, open and close files, etc., and don't forget to sign your public key.   

gpg --batch --gen-key <<EOF
Key-Type: RSA
Key-Length: 8192
Name-Real: NAME
Name-Comment: COMMENT
Name-Email: EMAIL
Passphrase: PASSWORD
EOF