The Rambam brings down in Hilchos Bikkurim that bikkurim has no shuir, but the Rabbanim say to give 1/60.
The Yo’ir Kino brings proofs for this number of 1/60.There is a gemara in kesubos(111b)which says even a barren tree in eretz yisroel will produce two donkey loads of produce.We then have a gemar in B.M(80a) which says a donkey load is 15 sa’ehs.The mishna in Bikkurim(1:11) says even if one has only one tree they are obligated to bring bikkurim.Lastly, we have a mishnah in keliem (12:3) that says a tenneh(basket)has a capacity of 1/2 of a sa’eh.
So if one has only one tree the minium amount of produce he will have is 30 sa’eh (two loads of 15).Then when the Torah says to bring a tenneh which is a half a sa’eh the amount comes out to be 1/60.
The Baal Hatturim says the pasuk alludes to 1/60:1)the word tenneh equls 60.2)samech =(60) is the only letter missing from the parsha of bikkurim .