try
EggSpriteSheet:scale(xScale,yScale)

Thank you very much! It works.

scale works, yes.
but the parameters are in fact percentages.

EggSpriteSheet:scale(0.5,0.5) -- half...
EggSpriteSheet:scale(2,2) -- double size

but I want to set the width and height directly to let's say 200 pixels

sprite.width = 200 -- not working.

is this possible in some other way?

-finefin

For some reason, no.
So you need to be mindful of the original sprite size.
If your sprite starts at 100, and you want it to be 200, you must set a scale of

newsize/originalsize

eg 200/100 = scale of 2

if you want 46 pixels, its 46/100 = 0.46

You need to be prepared to accept that in some cases, the maths will generate 'half a pixel' in which case it is hard to predict the exact size you will get. This can lead to one pixel gaps between objects placed next to each other.

yeah thanks. a little bit of math changes everything ;)

here's a small demo I made for testing your formula.
In fact I was trying to set the original size on an scaled object.
In that case it's originalsize / newsize of course.

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local boxOrig = display.newRect(  120, 30, 15, 100 ) 
boxOrig:setFillColor ( 155, 155, 155  )
 
local box = display.newRect(  120, 50, 15, 3 )
local boxOrigWidth = box.contentWidth
 
local function grow ()
        local randomScale = 1 + (math.random (10) / 10) 
        box:scale ( randomScale, 1 )
end
 
local function setToOriginalSize ()
        local boxNewWidth = box.contentWidth
        local scaleFactor = ( boxOrigWidth / boxNewWidth )
        print ("boxOrigWidth: "..boxOrigWidth)
        print ("boxNewWidth: "..boxNewWidth)
        print ("scaleFactor: "..scaleFactor)
        box:scale ( scaleFactor, 1 )
end
 
timer.performWithDelay ( 100, grow, 10 )
timer.performWithDelay ( 1500, setToOriginalSize,  1 )