I would like to display the size of my emails in the inbox. I right click and choose “Columns Configuration” then check to display the [Size] column, yet nothing changes in the inbox view. The size of my emails is not displayed. 
That is because you are using compact layout of message list. Open the menu Tools - Settings - Mail and disable the Compact layout or hide Sidebar and let me know if it helped.
Thank you very much. That resolved it.
Is the [Size] column removed in Ver7?
Size is still available, but it does not seem to be an option in conversation mode. I was able to use it in conversation mode with the “Show conversations in message detail only” setting activated. However, before I could see it in that mode, I had to turn conversations off, select size as a column and switch back to conversation mode. The “Conversation view in all views” apparently does not support a column for size.
It makes sense anyway; in conversation view what would be the “size of a conversation”? 
Have followed the instructions above (Menu, Tools, Settings, Mail, Message List, untick box with Use compact layout and radio button set at Always use single-line layout) but there doesn ́t appear to be an option to add a Size column to toolbar in my free version of eM Client. Can you confirm please?
The size column is not available as an option when viewing conversations.
To disable the conversation view, go to Menu > View > Conversations > Disable conversations. Then you can right-click on the column header and choose Columns Configuration.
It should already be in the right column by default, but if not you can add it from the left one.
Thank you, I did as you suggested and I can now see the Size of the mail including the attachment. This is important for me as I delete the largest attachments first to avoid using too much hard disk memory.