For a roll folded leaflet, in order to create artwork for a A4 folded to DL, or 6pp DL leaflets you need to divide up the 6 panels, creating 3 vertical panels each side. Imagine the document is flat A4 landscape, and you are looking at the outer cover side, so what you see is document which is 297mm wide x 210mm high (without bleed). Measured from the left to right, the left hand panel is 97mm wide, the centre panel (back cover) is 100mm wide and the right hand panel (front cover) is also 100mm wide.
The opposite is then the case on the other inside spread. Again reading from left to right. The Leaflet hand panel (inside front cover) is 100mm wide, the centre panel (inside of back cover) is also 100mm, then the right hand panel is 97mm. By having the flap panel slightly shorter on its width allows it to slide within the fold neatly without obstruction. The term ‘roll folded leaflets‘ is trying to describe the way in its its folded, the ‘flap panel’ is rolled over the next panel and then the next panel before its flat folded.
Don’t forget to add ‘bleed‘ to these measurements.