Number one: safety
Everything is secondary to safety of any practice. With open fire this is definitely very important. You don't want to start a fire to your home or to the forests or wherever you are going to perform the ritual. So choose proper place, have plans for fire getting out of control, know how to extinguish it fast and safely.
With that explained, you can focus on all the rest.
- size of burned offering. The smaller, the better, especially indoors. Even burned offering of the size of cut off nail, offered to fire using tweezers is enough.
- material and size of the bowl. When I burn my offerings, I use candle on the plate because I burn very very tiny stuff. (Still it makes smell in my room) If possible, get metal bowl. The obvious reason is that metal doesn't burn. However again safety tip: even just the heat from fire can cause a lot of damage. Consider getting one or few layers of ceramic tiles under the bowl. They can break, but will also hold away the heat... A big stone also works. Ceramic bowls work well, but they have limits on heat, so again consider how big offering you want to burn.
- shape. The bowl for burning should have some height to the walls of it. Prevention of burning stuff getting out of the bowl is the reason, Again, better safe than sorry.
- endurance. Especially important, if you want to use bowl over and over again. Nothing lasts forever, but you don't want to go too hard on your bowl, especially ceramic one, (breaking of offering bowl during offering is not pleasant experience)
Also consider:
- location. You better think well about where to use open fire to burn anything. many practical reasons for that.