Working on a project that I owe the inspiration for to this recently published project by fridolinux, it occurred to me that one of the legacies from Scratch - that influences how we think about changing a sprite's size (and similar) - is the fact that a relative (percentual) change in primitive blocks is not supported in equal manner as an absolute one is, although maybe, at least in the case of a sprite's size, imho, a relative change would be preferable as the default way of (thinking about) changing it.
Do you happen to agree, or disagree; what are your pro et contra arguments?
If you happen to agree, should those primitive blocks that as a legacy of Scratch only offer an absolute change, be replaced by a primitive blocks offering both ways of change, with a default one still the absolute one to ensure Scratch projects converted would continue to work in Snap!, too?
and so it'd be hard to justify breaking everyone's existing projects, or hairing up SET SIZE by including a relative/absolute menu input.
But in your project you should definitely build and use that custom block, rather than trying to figure out the right input to SET SIZE each time. A blessing of abstraction.
I could imagine changing the block's title to SET SIZE TO __ % OF ORIGINAL but there's zero chance of Jens going along with lengthening a title. :~)
But the big picture is that there are so many cases like this in which we could add inputs to blocks to make them please everyone. Maybe the best example is the GET/POST block in the Web Services library. It'll do exactly what you want, but nobody would know how to use it. Think about adding a degrees/radians input to all the trig functions, or a math/orienteering input to the direction blocks. Oh and for that matter, degrees/radians in the direction blocks too!
Make a library, just for yourself, with all the blocks you wish we had. :~)
(this time myself)