Accurate measurements are ESSENTIAL!

Sounds obvious, but if you think about how different sizes fit in different shops, just saying small/medium/large/ish really does leave it a bit at risk. 

Childrens' patterns are often more reliable, and usually fit without too much adaption. But for older kids and adults, chest and/or bust (around the widest part), body length (nape of neck to waist), arm (shoulder to wrist), waist and neck are all quite variable person to person. 

Although good quality wool is stretchy and reasonably forgiving, patterns pulled out of shape don't look so good! 

Knitting things like hats and scarves and bags is a bit more predictable and generic. Toys always come up smaller than you expect once you've seamed, turned them inside out and stuffed.