I see that a shape data cell/field can store a string of up to 64kb:
https://bvisual.net/2015/07/24/formatting-shape-data-in-visio/
Can a shapesheet user cell also store a 64kb string (obviously a 64kb string as a result, not a formula although that would be impressive for a formula length! 8) )
Thank you for your help!
Yup it is 64kb you can verify this with the code below.
(select a shape with the User.visVersion cell you can add, Microsoft's Basic Block shapes have this field in Visio 2019 by default)
Sub CramALargeStringIntoUserCell()
Dim zerototen As String
zerototen = "0123456789"
Dim largestring As String
Dim Counter As Long
For Counter = 1 To 6553
largestring = largestring & zerototen
Next Counter
Dim SomeCell As Visio.Cell
Set SomeCell = Application.ActiveWindow.Selection(1).Cells("User.visVersion")
SomeCell.FormulaForce = Chr(34) & largestring & Chr(34)
End Sub