Hi

>>>>>In model space when i select a dimension its scale enlarges and when deselected it goes back to normal again.
Looks like your current dimension style is Annotative, if you don't need ut to be Annotative, then go to Dim style properties -> Fit tab -> "Scale for dimension features" -> uncheck "Annotative". To remove annotative property from existing dimensions - select dimension and uncheck "Annotative" in "Annotation scaling" section in Properties palette.
>>>>. Could anyone guide me to what is the setting causing this for this one door block so i can amend it to remain the same scale when i change the view port scale like the others remain the same?
The door block also seems to has annotative property enabled - check this block by opening it in Block Editor and looking in "Block" section in Properties palette.