There's a good chance that functionality isn't yet available within Enterprise. Updates to ArcGIS Online come out more frequently than Enterprise. The next Enterprise release (10.9.1) should bring parity to the Map Viewer (Beta).
Thanks for the reply. Wouldn't ESRI consider the ability to change symbol sizes one of the bare bones, bare minimum functions for any Map Viewer product they release? I could understand not having some of the more complex functionality available yet but this blows my mind. This forces you to do initial map configurations - or changes - in Map Viewer, which according to ESRI documentation will cause any saved Field Maps form configurations to be completely lost. So, essentially, if you create a form in Field Maps, you are locked into not changing your symbology sizing without having your form wiped clean since that can't be done in Map Viewer Beta. We're forced to use Beta to keep our Field Maps forms but we're prevented from adjusting one of the most basic settings of a map. This is quite the corner they've painted us into...
Just upgraded to 10.9.1 and it does not include this ability either. Its unbelievable to me that the point size can't be changed. I agree that this should be a bare-bones, minimum function.