Most Oil combi boilers are larger than gas or LPG boilers and the majority are floor standing. Although you can get wall mounted options if required.
The best 2 manufactures for Oil are Worcester Bosch Heatslave Greenstar 2 and Grant Vortex Range. They range in size from 12kW to 70KW.
There is a definite trend to move internal boilers from kitchens and utility rooms to external locations. There is more work involved and the cost can be higher there are many benefits to moving to an external location.