Is 10x15 the same as 103 × 150 mm?

Almost. Many 10 × 15 cm thermal labels measure 100 × 150 mm. A DHL label needs at least 103 × 150 mm, even when rotated.

Why doesn't Shipnovo shrink the label to fit?

Shipping labels must not be reduced or cut off. Otherwise, the barcode may become unreadable.

No. Central formats apply to new API labels. PDFs already created keep their original format.

The printed label is reduced, cut off or has a wide margin. Usually, the paper in the printer does not match the selected format.

The app shows these messages in German. They are quoted exactly so you can match them.

The selected paper is smaller than the label. A DHL label needs at least 103 × 150 mm; an Internetmarke needs 89 × 43 mm. The shipping method settings also show these requirements.
A scaling option such as Fit to page is enabled in the print dialog.
The shipping method's format does not match your printer.

1
Check the central print format
Open Settings → ‘Shipping methods’, then ‘Label printing and DHL stock’. Under ‘DHL print format’, choose your label size, for example 103 x 150 mm.
2
Use custom paper dimensions with a margin
If you choose ‘Custom paper dimensions (mm)’, the paper must fit the entire label. Shipnovo rejects formats that are too small.
3
Print at actual size
When printing, choose your paper under ‘Paper in printer’. If it is too small, ‘· too small’ appears next to it. In the print dialog, choose ‘Actual size’.