Is there anything I can do to grow larger Blackeyed Susan flowers?

You can also call your local nursary for more information about your region and project. I use them all the time, quite useful.

Sun and fertilizer. See if this helps any.

https://www.almanac.com/plant/black-eyed-susans

Too much fertilizer and you I'll have lush green plants and few blooms. Or it could be the variety of black eyed Susan you have.

Good Organic Compost. Their are so many varieties of Black Eyed Susan(around43?) average flower size is 2-3" but depending on weather they(& Mother Nature) will grow different each year. Some years you may get less but larger some years lots of smaller.Just depends like I said on weather plants know how many seeds they need to produce to save themselves from upcoming weather conditions.Also many plants revert back to original blooms/ seeds instead of the fancy hybrids that were planted first so they would be smaller and different color,much more natural to the environment. Just add good organic compost to soil this Fall after they completely die back,don't remove them til next spring(they do better when not stripped away,left standing thru winter) then chop up & add them to compost. I say leave them alone and enjoy their beauty. The only major requirement for growing Rudebeckia is that it has plenty of sunlight. Some species can tolerate a few hours of shade, but these flowers are at their happiest and healthiest when they’re in full-sun conditions.

Take note that these flowers don’t do well in containers. A thriving root system typically reaches depths of six feet or more, and they are far happier when in the ground than in almost any container.

Tolerant of many soil types as well, the only time the poor land daisy might suffer is in very poor soil. They thrive in areas with more organic material, in conditions that are moist and well-drained.

When you consider the native habitat of a meadow, it’s easy easy to understand their desire for plenty of organic material and a lack of wet feet. Adding Red worms,and Mycorrhizae to soil is very helpful to get good healthy soil which produces healthy plants. Mycorrhizal fungi allow plants to draw more nutrients and water from the soil. They also increase plant tolerance to different environmental stresses. Moreover, these fungi play a major role in soil aggregation process and stimulate microbial activity. remember healthy soil produces healthy plants doesn't mean add a bunch of stuff you don't even know it needs. Adding Red worms(lrg or small and Mycorrhizae to soil is very helpful to get good healthy soil.