Link to mod (http://custom.simplemachines.org/mods/index.php?mod=1067)
Can anyone help me to code this mode so users have option to set size of thumnail?
Have you looked at how other bbc like the quote bbc and even the regular image bbc which accepts width and height attributes?
You could simply apply this to a copy of the bbc (So that way the orignal way works but so so specifying widths)
I didn't expressed myself right... I need option in admin panel to set resized image size... I tried to copy code from other mods and edit them to work with mine, but failed...
Well SMF has that ability already built in:
AdminCP->Attachments and Avatars->Attachment Settings=>
Maximum width of thumbnails:
Maximum height of thumbnails:
This is not for thumbnail in attachments... This is for my mod Image Thumbnail
Why don't you just use those settings defined in that section to resize the images in your mod? Or do you want every user to be able to define their own. Then it would be best to do it like Sleepy said and allow them at attachment time to define it on the fly.