If I understand your question correctly and you were in reference to an ArcGIS Online map you have published, yes. The pop ups are substantially configurable and even support some HTML and Arcade code. Although size is not directly controllable you can modify the content which indirectly affects the size. Take a look at:
You can customize the Esri popup size through its CSS property's. Simply use the following code in your <style></style> and change whatever you wish to your liking and add or remove different CSS properties:
