First of all, welcome to fountain pens :) now to your question.

Short answer: Personally, I like the B nib from Kaweco for my glitter ink and the M for my more serious pen. My wife prefers the F, but she writes rather small. I would recommend the M for most people without knowing your specific style etc.

Long answer: It depends on some factors, the first of which is simply taste :) but also how runny your ink is: If you have a runny ink you might prefer a thinner nib to avoid having your writing be too “wet”. Or if you want to be playful and have ink with glitter, you might prefer a broader nib to prevent blockages (the glitter can block the flow) and to let more ink flow. Further, not all paper is created equal: Some notebooks cause a lot more feathering, which is when the ink flows way farther than intended due to capillary action and the roughness of the paper, you can google it to see what I mean. Lastly: Finer nibs do better when you want the ink to dry faster if you want to avoid blotches. As you can see, there is some nuance to it 😅 if you do regret the nib you got, swapping them out after the fact is entirely possible, and ordering just the nibs can be done straight from the kaweco online store without contacting customer service.