scale up or down DIN sizes automatically

satyja,

"I want to export 300ppi JPG files."

I believe this implies use for such print where 300 PPI JPEG is required, otherwise it is better/necessary to do it in another way, either avoiding JPEG/other raster formats, or forgetting about the resolution in favour of (closest integer values of) actual pixel x pixel sizes.

In connexion with print where 300 PPI JPEG is required, and presuming you are scaling between A page sizes, you can scale up/down to the actual W/H size of the target A format (in mm), which can be done with the legacy Save for Web to ease the pain.

Or you can use these simple facts for scaling while exporting to ease the pain a bit:

1) When you scale up/down between even A formats or between odd A formats, it is very simple because the scaling factor  corresponds to the number of steps down/up in the denomination.

In other words, you double the scaling factor/size each time you go 2 steps down, and halve the scaling factor/size every time you go 2 steps up.

Based on A4 or A5 you have,

A0: x4               A1: x4

A2: x2               A3: x2

A4: x1               A5: x1

A6: x0.5            A7: x0.5

A8: x0.25          A9: x0.25

2) When you scale up/down between an even A format and a neighbouring odd A format, the scaling factor is based on the square root of 2, so you scale up/down by that factor when you go 1 step down/up.

Based on A4 or A5 you have (similarly for other neighbouring A formats):

A3: 1.4142x      A4: 1.4142x

A4: 1x               A5: 1x

A5: 0.7071x      A6: 0.7071x

Edit: To avoid/have the pain only once, you can do as Ton said.

If you want to save yourself some dimensions/resolution calculations, I would just setup some artboards.

You can scale them while exporting. Depending on which route you take exporting you have to set this up via the scale factor or the resolution.