The possessive suffix attaches itself to whatever's the last part of the noun phrase, right? So, "my big oranges" (with -ku) would be "jeruk-jeruk besarku"?
Yes , that's correct.
The English word order in a noun phrase is mirrored compared to the Indonesian word order.
The possessor takes the last position in the noun phrase.
[oranges] [big] ==> big oranges.
jeruk-jeruk besar saya
[oranges] [big] [I] ==> my big oranges.