So a few more questions:
What tooling do you have or plan to use?
Bandsaw big enough to cut body profiles?
Hand tools like knives, draw knife, spoke shave, hatchet or rasps?
Power tools like a foredom or dremel?
Are you creating your own patterns or going by a produced pattern?
I love making wood ducks, it looks intimidating but I found it's not bad, the worse is any puddle hen. If doing hens I find that
@Steve Sanford method of painting works well for creating the illusion of feathers and patterns without the tediousness that is ideal for gunning birds. He has a website with paint tutorials and layout that you can visit.
A Butterball painting tutorial The Bufflehead – what I learned as Butterball on Great South Bay – is a beautiful bird. I have carved several “mantel piece” drakes over the …
stevenjaysanford.com
As far as paint, you will see that Steve uses Behr paint. Rustoleum is fine paint as well, I would just use a low sheen for decoys that are smooth surfaced, and even for cork.
Like William, said if you would like to do flocking then the glossier paint works well on that.
If you want to learn more about the flocking and processes to that then Don and William have done a fine job detailing it here.
Thread 'All Things Flocking'
https://duckboats.net/community/threads/all-things-flocking.355931/
Some members use oils and some use house paint and others, like myself, use acrylics like Golden brand. Each has its nuances and are all great ways to go.
You can see some oil paint and nice carving work from
@jode hillman at this workbench.
Thread 'Late......June workbench'
https://duckboats.net/community/threads/late-june-workbench.355724/
When dealing with size of birds: I don't think it's that critical depending on species. For puddle ducks I personally think going 2x on the size does no harm. So having teal the size of a mallard or maybe even a magnum size mallard won't be an issue unless you have very large over sized mixed with life size IMO.
For divers and sea ducks, the bigger the better. You want them to show up on big water and mixing of sizes does no harm in my experience. Blacks and whites are a big deal as they are very visible so it's okay to make those areas more dramatic in size compared to life.
Few this to see
@Troy Fields massive eider.
https://duckboats.net/community/threads/big-eiders-on-the-way-edited-with-the-final-6.351453/
Dealing with style or patterns, I personally like doing realistic and making it look like a real bird but it's tedious and the perfectionist in me can cause me to get hung up on non important details. The are styles to areas that I also like and have made that as well such as the Core sound and Currituck sound style of birds. I have also made wire and canvas which are pretty simple and a joy to make.
If drawing your own patterns you can really make it how you want, one of our members
@patrick mccarthy has a very unique and wonderful looking birds that can be viewed here.
https://duckboats.net/community/threads/march-work-bench.355544/post-365544
And there are a lot of other great examples of where you can go with this throughout the forum. If you need help working on a decoy pattern or paint schemes then don't hesitate to ask.