In this case, size DOES matter! I would only risk it if your betta is one of the more mellow one, and if the guppies are plain. I have seen my fancy delta tail gups get really nippy at each other. And the larger finnage could cause the betta to think of fighting that rival.
My guppies are in one tank, and one of my bettas is in the tank next door. Sparky (betta) will see one of the delta tails and flare at him. Or he will see Goldie, my huge female guppie (she's as big as the betta!) and flare at her.
If you had a huge tank, you might try it, but only if you had somewhere to move your betta if there are problems. You would need plenty of plants (I prefer fake) and decorations to create hiding spaces for all fish involved.