If its just a normal e30 the speakers are 13 cm (5.25 inch) front and rear. I kept to the standard size and bought good qaulity new ones (Vibe front and back), I also soundproofed the whole rear shelf and bolted 18mm MDF pieces under the shelf to give me a solid mounting for the speakers. Soundproofed as much as I could in and around the kick panels, I'm very happy with the way it sounds !
Been thinking about this for a while and the views Ive had were to replace with better quality standard sized speakers unless you specifically want to get bigger speakers (6x9s)