So Universal has three main large parcels of land all which can support a theme park and hotels.
There is the North Eastern SLRC 245 acres.
The main area is 654 acres (not including the 101 acre plot dedicated for the warehouses).
Lastly you have the South East large plot covers 183 acres. (Edit: I stand corrected its actually 207 acres)
That still leaves 3 plots left over (Convention center parcel is 23 acres, southwestern isolated I-drive plot next to top golf is 20 acres, plot on i-drive next to Lockheed Martin is 17.25 acres). The I-drive plots can manage to tie to the main plot if Universal were to obtain two more plots of land. (UCPM plot between the two Universal owned I-drive plots and the one located next to Ripley's would give them 56.4 additional acres).
For point of reference:
The Royal Pacific Resort (the largest Universal Resort) is 53 acres. (of note: that also includes the retention pods and parking garage)
The Endless Summer Resorts covers 47.54 acres including the parking garage but 10.4 acres of the plot is unusable land in the lake)
The Two Guest Parking Garages cover 69 acres.
Citywalk 1.0 is 35 acres
IOA is 101.1 acres including backstage areas
USF is 128.3 acres including park admin buildings, the sound stages, and Universal Studios Administration facility.
Universal Employee parking is 24 acres.
Volcano Bay is 31 acres.
Adventura Hotel is 6 acres.
Each one of those three large parcels could fit one Royal Pacific sized resort, 10 acre parking garage, and one IOA sized park if Universal really wanted it. (North East plot would still have 81 acres left, South East plot would have 43 acres, main plot is enormous so obviously it can fit).
With this even in regards to the 4th park, Universal has tons of options in regard to the scale. Even if they did two 150 acre parks on the main plot with a 50 acre water park, 50 Acre parking area, 50 acre city walk, 5 resorts each with a higher limit of 50 acres: they would still have 54 acres remaining. Don't get me wrong the areas would and design would be weirdly shaped to fit all of those things but they still have that option when developing. Even they could do a Beijing sized park (300 acres) on the main plot as
@Griffin C has shown above and still have options for later.
(Source: Orange County Property Appraisers office)