I'm sorry. After I had written the post above I've found my mistake. Have forgotten to initilaze the fields in the array. The following works:
Type MyType Field x Field y End Type Local a:MyType[200] For i = 0 To 199 a[i] = New MyType Next count=0 For Local b:MyType = EachIn a count=count+1 Next DebugLog "Size: "+count End