>>kstrau+Ug
`9↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑9` seems like a reasonable guess (barring encoding cheats/trickery like @masfuerte commented!)
Edit: I've misread the above comment and my number is is 64 bytes (significantly more than 64 bits. The largest 64 bit number through my approach would be `9↑↑↑↑↑↑9`, which is significantly smaller.
>>tuhgde+9V
In terms of the Fast Growing Hierarchy, it's about f_62(9) or what the article would denote as [62] 9. It's way smaller than Graham's Number, which involves 64 iterations of mapping n to 3 ↑↑↑... {n uparrows) 3, whereas this expression has between 1 and 2 iterations.