[Jackmlgd]

Hi guys, this has probably been covered many times before but I have looked and I can't seem to find a straight answer.

I have a 2016 BMW F20 and the stock speakers in it are awful. I have combed the internet (albeit probably not very well) and I can only find speakers that are plug and play - these are too pricey for me as I cannot justify spending £200/£300 as I don't spend too much time in the car. I have 2x 4" Hertz speakers from a previous car but they are not plug and play and I would not a clue how to fit them. I also cannot find anywhere where it says what size speakers my car takes. I only have the front door speakers, the under floor speakers and the parcel shelf speakers if anyone would know what size they are.

So I guess my questions are:
1.Can I get any speakers as long as they are the correct size and how would I connect them to the blue connecter in the doors?
2. How would I fit the speakers under the floor and in the parcel shelf?

Thanks,
Jack