Buying my first back bag - what size do I need?

[Watcher]

I have a Savage in a Dolphin stock. A pal has loaned my a Shooting Shed bipod so I need to buy a back bag. Although I can pester the lads on the range to try theirs what factors do I need to consider in terms of sizing? Looking at the Dolphin stock I've got a lot of latitude when it comes to adjusting the back plate; does that mean I can just buy any bag? 

TIA 

Matthew

[Kipper]

Is it for Fclass

Simon fox is your man if so. They stock various ear sizes etc of sebs rear bags and price can't be beat.

 

Start at £60 and go up to £168 empty but also sells the sand.

Ear size is important (measure your rider) and come in low or high depending on your preference. 

[Kipper]

https://drive.google.com/drive/folders/1u6njOmdbGT5Q2Wd77TFjxOz_x3HP9U6P?usp=sharing

[Watcher]

Thank you.