I don't know where Excel stores the default height and width for shapes. I assume by default you mean the size of the shape when you click to place rather than drag to size. Ovals, for instance, are 72x72. Same for squares.
One way you could do this is use the AlternativeText property of the shape. You could store the default sizes in this property. Right click on the shape, choose Format Autoshape, go to the Web tab, and type 72|72. I'm using the pipe as a delimiter between width and height. You'd have to figure out what the default size is for every type of shape you have, but like I said I don't know where Excel stores it. Once you have the AlternativeText property set, you could use code like the below
Sub FixShape()
Dim shp As Shape
Dim vaDefault As Variant
Const sDELIM = "|"
For Each shp In Sheet1.Shapes
If Len(shp.AlternativeText) > 0 Then
vaDefault = Split(shp.AlternativeText, sDELIM)
shp.Width = vaDefault(0)
shp.Height = vaDefault(1)
End If
Next shp
End Sub
This will set every shape that has something in AlternativeText to the width and height you recorded. This assume you're not using AlternativeText for something else.
If you don't want to use AlternativeText to store it, you could hardcode the values in VBA
Sub FixShape2()
Dim shp As Shape
Const lDEFOVALHEIGHT As Long = 72
Const lDEFOVALWIDTH As Long = 72
Const lDEFSQRHEIGHT As Long = 72
Const lDEFSQRWIDTH As Long = 72
For Each shp In Sheet1.Shapes
Select Case shp.AutoShapeType
Case msoShapeOval
shp.Height = lDEFOVALHEIGHT
shp.Width = lDEFOVALWIDTH
Case msoShapeRectangle
shp.Height = lDEFSQRHEIGHT
shp.Width = lDEFSQRWIDTH
End Select
Next shp
End Sub