A larger / taller SMB can be an advantage *IF* it arrives at the surface completely inflated, and *IF* the diver can keep it upright. A 6'ft SMB that's 1/2 full and requires 1/2 of the that 1/2 be pulled under water to keep it upright is less effective than a full 1 meter smb that has 90% of it out of the water.
If one launches any SMB from significant depth there is a good chance it will be full at the surface, much less so from shallow depth. Being able to fill a large SMB when shallow and maintain depth requires considerable practice and skill.
A large SMB can also require the diver be significantly negative in order to generate the force required to keep it up right.
What's easier to see? An upright 1 meter bag or a 1/2 full limp 2 meter bag lying on the surface?
Tobin
How do you know what it takes to keep the SMB up right ?