We've heard the rumors before so the report from the reputable Korean media ETNews comes out as a confirmation about the upcoming Galaxy Z Fold3's and Z Flip3's display sizes.
According to the inside sources, the Galaxy Z Fold3's external display will measure 6.23" in diagonal while the inner, extended display will go up to 7.55". In other words, there won't be any changes in size, just expect a more compact overall device.
The Galaxy Z Flip3, on the other hand, will get a larger 1.9" external display, coming from 1.1" on the original Z Flip. The main, inner panel, however, remains at 6.7".
Other details were not mentioned but we still have a little over a month until Samsung unveils both devices so more rumors will pop up until then.
Source (in Korean)