First of all, I apologize for my English—it's not my first language.

I want to create a weapon that can cast a spell using charges. The explosion size should depend on the number of charges used:

• 1–4 charges create a 5ft explosion,
• 5–9 charges create a 10ft explosion,
• 10 or more charges create a 15ft explosion.

Is there a way to make this work?

So that the player can decide how many charges they want to use?

You can write that as the description of the item but you cannot automate it for virtual tabletop systems. It would just be words written down in the item description.