To an extent, it doesn't really matter lol.The Laylax SP 190 Spring does have an inner diameter of 8mm. I have spring guides for 7 and for 9 mm, so i have to make one can mod between spring and one spring guide and one between spring and piston.
Is that better, than a 7mm oder 9 mm spring that fits exactly on the inside or on the outside?
9X13mm springs tend to get more power for less pull, but it doesn't really mean you should change from what you have as the difference isn't too important.
Also, if you can, try to get some heat shrink tubing to put on your 7mm spring guide instead of aluminum can, it will help a little bit with sound.