Not sure how to change the extent of this layer. You might consider using IFeatureLayerDefinition.CreateSelectionLayer instead - the resulting layer has an extent based on features that it contains.Option Explicit
Sub Test()
Dim pWSF As IWorkspaceFactory
Set pWSF = New FileGDBWorkspaceFactory
Dim pFWS As IFeatureWorkspace
Set pFWS = pWSF.OpenFromFile("D:\Projects\SAWS\data\EncinoPark.gdb", 0)
Dim pFLayer As IFeatureLayer
Set pFLayer = New FeatureLayer
Set pFLayer.FeatureClass = pFWS.OpenFeatureClass("EARZ_ZONES2")
pFLayer.Name = "mylayer"
Dim pQF As IQueryFilter
Set pQF = New QueryFilter
pQF.WhereClause = """EARZ_ZONE"" = 'AREA 5'"
Dim pFSel As IFeatureSelection
Set pFSel = pFLayer
pFSel.SelectFeatures pQF, esriSelectionResultNew, False
Dim pFLD As IFeatureLayerDefinition
Set pFLD = pFSel
Dim pLayer As ILayer
Set pLayer = pFLD.CreateSelectionLayer("mylayer", True, "", "")
Dim pMxDoc As IMxDocument
Set pMxDoc = ThisDocument
pMxDoc.FocusMap.AddLayer pLayer
End Sub