Myhermes.co.uk : a maximum combined length and circumference of 225cm

max weight : 15kg
No side longer than 120cm

Calculation :

((width + height) x 2) + length = < 225cm. 

myhermes explain : link
myhermes site general info : link

If you use myhermes.co.uk to send parcels, you may wonder what this means :

Any parcel must also be less than 120cm on any side and a maximum combined length and circumference of 225cm.

If you are sending a parcel with no circular measurements, you may wonder how to equate this to a cube/rectangle package

The answer would seem to be in the page about 'check parcel size', it says this  :

We can carry parcels that weigh up to 15 kilo's each, up to 120 cm in length. If you add the two shortest dimensions and multiply them by two and then add the length, the total calculation needs to be fewer than 225 cm. 

link to myhermes.co.uk measurement answer

Summary of the 'size' rule when sending a parcel in this category :
1. First, you need to check no side is greater than 120cm long.

2. Then you add the 2 smallest sides and double it.
Then add that number to the longest side.
This total must be less than 225cm

MyHermes maximum size Calculator

First Check : no side longer than 120cm

MyHermes Calculation
smallest side (cm)
+
next smallest side (cm)
x 2
+ longest side (cm)
must = less than 225cm

Solution

Example :
Package 80cm x 20cm x 15cm

1st step test
All sides are less than 120cm so pass first step.

2nd rule test : 
Add 2 smallest sides : 20 + 15 = 35cm
Multiply this by 2 :  35 x 2 = 70cm
Add longest side  : 80 + 70 = 150cm
Is total less than 225cm ? 150cm means YES

Minimum package size : Also they say a package should be at least A5 size (to get the label on), but I think you can perhaps shrink the label size in the printer, so you would be best to check if you can send packages under A5 size.