Compost to a gardener is like black gold. Most keen gardeners visibly start to quake with excitement once you start talking about compost & COMPOSTING!
Here is a picture of my compost bins - made out of old pallettes (God created pallattes just to use on the allotment garden, and especially for compost making).
Raw kitchen waste, grass clippings, old vegetation when clearing up etc. and anything else that will rot down gets put in the bin on the left. When it's full the contents get tumbled into the middle bin. When the left side bin gets full again the contents of the middle bin goes into the right hand bin and the contents of the left hand bin gets tumbled into the middle bin. By the time the left hand bin is full for a third time the stuff in the right hand bin is brown, crumbly, odourless (well sweet smelling) and ready to be used as rocket fuel on the crops.
Go to my web-site at aberaeronallotments.org and click on the "Basics" tab and then select the "Compost" tab for more detailed info.
As for the size of the gaps. God made pallettes and designed them exactly the right size with gaps to allow plenty of air through your compost heap - making sure the aerobic bacteria have plenty of oxygen to work with. Without gaps there is not enough oxygen and the anaerobic bacteria take over, that's when you'll need a clothes peg over your nose. If your compost heap smells foul it usually needs a bit of warmth and OXYGEN!
The roof is also very important, it keeps the rain off. Sodden compost material also smells and goes all slimey (ach a fi!). It needs to be damp (and warm) but not soaking wet.
Regarding rodents. You should never put cooked food and meat in your compost bin because it is just that - FOOD - it will attract rats and other bin foragers. Rodents DON'T eat compost! You can put vegetarian manure in (cow/ horse/ sheep/ rabbit dung etc.) DO NOT put carniverous droppings in (dog/cat/pig droppings etc.) because of the risk of parasitic contamination (tapeworms and various other nasties). The trick is to put the correct material in, not the wrong material and then build it like Fort Knox to keep the furry little critters out!!
Does that help?