Hilchos Bikkurim 2:17

There’s no prescribed measure for first fruits under Biblical law. Under Rabbinic law, one should give one-sixtieth of his harvest, though he can designate his entire crop as first fruits if he so wishes.

Hilchos Bikkurim 2:18

If someone separated first fruits and subsequently added to them or decorated them, what he added also becomes first fruits. This is the case when one brought first fruits from Israel; if he brought them from Transjordan or Syria, the additions don’t become first fruits. Even though they’re not like the first fruits, they should still be eaten only in a state of ritual purity. Wherever they come from, first fruits should only be decorated using fruit from the seven species for which the land of Israel is praised.