Don't know if there is a better solution, but I did it!
B4X:
Sub changeImageSize(bmp As Bitmap, destWidth As Int, destHeight As Int) As Bitmap
Private imv As ImageView
Private cvs As Canvas
Dim pnl As Panel
pnl.Initialize("pnl")
imv.Initialize("imv")
pnl.AddView(imv,0,0,bmp.Width,bmp.Height)
cvs.Initialize(imv)
Dim rSrc As Rect
Dim rDest As Rect
rSrc.Initialize(0, 0, bmp.Width, bmp.Height)
rDest.Initialize(0, 0, destWidth*2, destHeight/2)
cvs.DrawBitmap(bmp, rSrc, rDest)
imv.Invalidate
Return imv.Bitmap
End Sub