Ive been thinking about this for a while and could use help

So I have been thinking of writing my own Harry Potter fic for a while, specifically making my own political!Harry fic. I have been unsure for a while because while I know that I like writing and I have got okay feedback in past, this feels like a massive undertaking and I wanted opinion of other authors: if one starts off writing first time for places like AO3, is it better to start with small one shots or rather something bigger? I have a feeling that if I start with this particular idea then it will either become multiple series or it will be huge 100 chapter fic. I simply a lot of ideas how to implement something different into that whole trope as I feel like it is a little bit underwritten part of the fandom sometimes.

Thanks in advance💕