Actualy there´s no better size, researchers of the world work with a gama of ranges. A few articles talk about this, one of them points no significant difference in richness of bees captured in pan traps with 4, 7, 10 and 12 cm and little or no effect in abundance in bees and flys. I reccomend focus on the height position of the trap compared to the vegetation around and use a more economic material.

Reference:

Gonzalez, Victor & Osborn, Alina & Brown, Emily & Pavlick, Cody & Enríquez, Eunice & Tscheulin, Thomas & Petanidou, Theodora & Hranitz, John & Barthell, John. (2020). Effect of pan trap size on the diversity of sampled bees and abundance of bycatch. Journal of Insect Conservation. 24. 10.1007/s10841-020-00224-4.

What are the best sizes for pollinator traps? Sometimes size is important, but color also influences trapping. There are several reviews about what is the ideal size, and how far apart they should be, like the one Carlo sent, here is a link where they comment a little about it, it might be a little old, but it will give you a general idea, pan traps as well as Vane traps, have a certain bias, to catch Halictidae, but still successful in your search. http://bio2.elmira.edu/fieldbio/beemanual.pdf

Recommendation

Pan and Malaise traps have been used widely to sample insect abundance and diversity, but no studies have compared their performance for sampling pollinators in forested ecosystems. Malaise trap design and color of pan traps are important parameters that influence insect pollinator catches. We compared pan trap (blue, yellow, white, and red) and Malaise trap catches from forests in three physiographic provinces (Piedmont, Coastal Plain, and Blue Ridge) of the southeastern United States. Similarities in trap performance between sites were observed with blue pan traps being most effective overall. Our results showed that various pollinator groups preferred certain pan trap colors and that adding color to Malaise traps influenced insect pollinator catches. However, pan traps generally caught more pollinators than Malaise traps. Because of their low cost and simplicity, using several colors of pan traps is an effective way to sample relative abundance and species richness of flower-visiting insects.

Thank you Carlo Benetti , Rattan Sharma and Raider Ayrton Castro Pino for your helpful suggestions!