There are a lot of possible answers to this question. It would help if we knew which flavor of RM you are referring to. RM2 has Elemental Companion which we use solely for all magic. We've never had to house-rule anything from it to make the spells work.
As for size, pea vs. ball, it was irrelevant. The 'vehicle' that transported the matter wasn't important. If the spell called for a 1' ball, then it was a 1' ball or equivalent. An air elementalist may want his 'ball' to look like an attacking falcon of air... still about 1' in size. The electrical elementalist may want his attack to look like a 1' x 1' electrical web. It was all aesthetics and allowed the caster to personalize his spells.
I never allowed casters to lob Ball spells to avoid detonation before arriving at its target centre of effect just for that reason. They flew straight from a hand(or eyes for flavour) to their end point.
We allowed Spell Mastery to make lobbing a spell possible. There is also Spell Trickery, another optional skill from a later RM2 Companion, that allowed spell manipulation.
in fact if the fireball were behind you the shield could trap part of the blast against your body making the shield a detriment not a bonus
Elemental Companion deals with this concept for cone attacks. If a 25' length cone hits a wall 15' away, there is 10' of cone that reflects back towards the caster (or at the appropriate angle). The poor bloke at the center of the cone AND within that 10' reflection section, gets pulped! Center of a cone attack is double damage. Center of the reflected damage and he gets horribly punished again.
Using your fireball example and a shield carrying target, you could easily apply a version of the cone rules to that single target.