I did the following (I guess it works all the time):
Dim blockObj As AcadBlock
Dim mtextObj As AcadMText
Dim insertPoint(0 To 2) As Double
Dim txwid As Variant
Dim txht as Variant
Dim txstr as String
Dim minExt As Variant
Dim maxExt As Variant
Dim junk as Variant
'set text data
insertPoint(0) = 0
insertPoint(1) = 0
insertPoint(2) = 0
txwid = 3 'sets text box width
txht = 0.25
txstr = "THIS PLACE IS NICE" 'sets text
'Create a text Object in the blocks model space
Set mtextObj = blockObj.AddMText(insertPoint, txwid, txstr)
mtextObj.Height = txht
'set ratio between text lines (1 = 0.66666 times height)
mtextObj.LineSpacingFactor = "1"
'Get bounding box info
mtextObj.GetBoundingBox minExt, maxExt
junk = Int((maxExt(1) - minExt(1)) / (txht * 1.66667) + 1)
MsgBox "MText lines = " & junk, vbInformation, "MText"