Speaker size

[Taliesins]

Hi all I know absolutely nothing about Audio so i need some advice.
I need some new speakers for my truck as the existing are buggered.
What size do I need. When I am looking at various sites speaker sizes seem to be in cms ie. 13cm or 16 cm etc. Which ones will fit in the existing mounts.
Thanks in advance

[Ace Piker]

Take the old ones out and measure them maybe?

[The Hornet]

I put replacement 4" in straight in the front and 5", with slight modification to bass box, in the rear side panels. They were Reisens from Maplins.

[POPEYE]

Be carefull with the fronts, use the original sound box which takes 31/2" (i think) speakers' i juncked the box and fitted 4" behind there and it was awefull, drummed right through the dash, as the boxes had gone with the binmen i fitted em in pods on the door, there fine now,
Also changed the rears for 6x9"