I know many people have had problems in the past with illustrator files being imported into C4d using the Merge command ending up the wrong size and not centred in the composition. I'll deal with the latter problem first.
In Illustrator by default the origin 0,0 is at the top left of the composition. If you want your splines to be at the centre of your C4D comp, then you need to first make the rulers visible in Illustrator, then click and drag the small square between the horizontal and vertical rulers and place it in the centre of your artwork. Save the file as Illustrator Version