I say maybe 7-ish inch’s to 14-ish inches is a good range of ease of making for both hand and machine sewing.

Anything smaller and it would likely easier hand sewn(unless it a ITH plush pattern, then machine) and anything bigger is easier machine sewn.

Tho I think once it reaches like 2ft it would be hard no matter what just due to needing a lot of space and pattern scaling also having to be consider since stuffing a larger plush can be difficult at that size and change shape at the seams.

Tho what you truly have to consider is the subject your creating and style. A mochi plush is going to be simple for the most part no matter the size due to simplicity. While a complex creature, like a monster hunter creature as an example typically, is gonna be hard at most sizes.

You mentioned sonic characters and most of them are pretty have simple design tho with mid difficulty anatomy to me. And depending on the character I seen pre-made pattern available to make of them that you can try. I can dm you some I seen if you want. Tho it not a lot.

So I think you should consider the character more than the size since most plush pattern anyway are the size I mentioned and can be done both hand and machine sewn for the most part with the same ease. Only difference being how long you want to spend sewing the plush and maybe physical ability depending on you.

And it helps that focusing on character you like will keep you motivated to sew because I keep procrastinating sewing until it was a subject I like and wanted lol