The right size for speedlight bounce card?

I'm making a bounce card for my Nikon SB-800, using a white foam sheet (with a black sheet for the back). For those who have used a bounce card, either bought a commercial product or made one, I wonder if I should make mine as large as possible, or if it does not matter much.

The foam sheet from a local craft store comes in 9x12 or 12x18. I know for a soft light source, the bigger the better, but we're talking practical use here.

I'm preparing to take photographs of children and adults dressing up this coming Halloween, going trick or treat around the neighborhood. It will be dark at night, outdoor at a small neighborhood park with one street lamp overhead, pavement and grass and a few bushes around, may be a tree or two.

With no ceiling or wall to bounce off, and a light stand is not practical as people will come and go, I need to move around, it's not like an event where people mingle and I can setup a spot for photos. I'm using my speedlight, I thought about off camera but I don't have an assitant, I'm still debating if buying a flash bracket is a good idea, please share your thoughts here too.

So I made and tested a couple of "diffusers" out of some translucent plastic bowls (instant noodles), the resulting light is too weak. (I know not to buy a Gary Fong bowl for sure now, haha!)

I had also cut up a white cardboard sheet and made a round bounce dish, about 7 inches in diameter. The result is much better light. So I'm making a more durable version with foam sheets. As I type this, I'm thinking I may buy both sizes and try both.

I see that some commercial versions are quite small. Just wondering if anyone can share their experience.