we had a relative similar issue on our Server regarding WB having a live Connection to an Oracle DB. So in this case, the Problem was not really the size of the WB but due to the WB design (users did not want to Change anything on this...) we had to split the number of Sheets in order to fit with the user's acceptance criteria's. One of the main differences you have between Tableau Server and Tableau Desktop is that when you open a sheet on T Desktop, it only proceed the metadata of the sheet. On T Server if the WB is published with tabbed Sheets, the full metadata have to be proceeded before Rendering... And for users it can be a no go criteria for the Performance....
Otherwise, regarding your question, I do not think that a 25 MB WB Need to be splited but the question is more what would be the acceptance criteria in terme of Performance for your end user...
If you look for this very usefull document : Design performant workbooks by Alan Eldridge, you will find a thousand tricks that can help you improving the Performance of your Content.
I hope it will help you.
Hi Ross. 25mb for a workbook size is not overly alarming, and neither is 5 dashboards; with the caveat that you are running on a decent server with sufficient speed in terms of CPU, RAM, disk, etc.
What is current rendering time? One possible analysis tuning option is to have a look at the performance recording as this may help you identify where the bottleneck is: Create a Performance Recording