How big is a UK Postbox slot ?

question : How big is a UK postbox opening/slot ?

older postbox ; smaller slot

Answer : It can vary (mostly depending on age). Probably a rule is ; the older the box, the narrower (and probably smaller height) the slot. Central post offices probably have the biggest slots.

The biggest slots are pobably in new areas and also at the local main post office.

Note : The slot slopes upwards
The slot tends to slope upwards (e.g. to keep out rain I guess), so if tight, it could be worth trying to push the parcel in slightly upwards (to follow the angle).

Can you put parcels in postboxes :
My understanding is YES. I have put parcels in.  

Estimate sizes :
An estimate would be between 8-12 inches wide and maybe 5.5cm high (or smaller).

Width : In old areas, it can be as little as 8 inches wide. But in the newest areas it may be around 12 inches wide. And areas between old and new may vary between these sizes. This rule probably applies to height as well.

Height : As mentioned above, the newer the postbox the more likely it has more height. My local postbox made around 1981 is probably around 5.5cm high.

I use a postbox probably built around 1981 which has a bigger slot than older versions (both in width and height)

Examples for 1980's postbox slot :
Using a circa 1981 postbox, I have found the following :

UK postbox slot (1981)	width	height
	20 cm  : fits easily 25 cm : fits quite easily large letter size (exactly) : should fit 1980+ slot easy	4.8cm high package : fits easily 6 cm high box : does not fit

Note : Older postboxes tend to have much smaller slots !