I use a 10 on the face, tummy and sanitary area. A 15 on the feet, until I get better then I will move to a 30.
I like Poppy very very short so I use a 5 blade on her body and also do her legs in a 5 in the summer. A 5 blade leaves 1/4" of hair and that is plenty short for us. We live at high altitude and are 5,000 ft closer to the sun than those near sea level so sunburn and stron UVRays can be an issue for humans and dogs. I like a bit of protection from that 1/4" of coat on Poppy.
In the winter I have gone a little longer, using a 3 blade on her body and legs which leaves 1/2" of hair.
Make sure you NEVER use skip tooth blades as they are for a different type of coat than poodle fluff and can make it easy to cut the dog with the blades. A blade which says the size and an f or fc designation is a finish blade and that is what you want. I have an old Oster clipper which takes the individual blades.
I also have a Wahl Bravura with the 5 in 1 adjustable blade and the stainless comb attachments. I can get all those lengths with using the blade on various settings and for longer lengths use comb attachments. In a Wahl comb attachment the dark purple comb is 1/4", equivalent to a 5 blade, and the tan comb is 1/2" equivalent to the 3 blade. Use the combs over the adjustable blade on any setting EXCEPT the 40 setting...it will break the comb if you use it over the 40 setting.
I am still a novice, but getting better each time. Poppy is very cooperative. Have fun!
Here is Poppy with her short body and legs, done with a 5 blade and floofy tail, ears and topknot. She is a tom boy and loves to play in mud, dirt and water so short works for me....easier to keep clean.
I use a 40 on face and feet, a 15 on the belly and the back of the tail and around the anus, and a 30 on the front legs and the sides of the tail. If I decide to put her in a full continental or modified continental again I'll use a 30 on her back legs and back end.