newbie needs help please

[ant]

hi all hope you passat peps can be of help !!!!,basically just brought a 51 plate 1.8t and the stereo is poop,had a veccy sri and i put a full install in myself every speaker and tweater changed and sub and amp fitted,so im not shy of doing the work but im a bit lost on speaker sizes and changing the head unit.i want to buy a new full set up but dont know what size the speakers are.can you fit 6x9s in the back shelf ?????what do i need cable wise to change the head unit, can i still use the 6 cd changer ? i dont want any gaps where the original stereo is so do all stereos fit perfect or do they sit out a bit gaps etc,anyone with any pics of there head unit sitting nice n flush and boot installs would be nice peps,cheers