As you are using vector graphics, the resolution doesn't really matter. It will matter if you export it to a bitmap or if you print it (as a bitmap, but you can also print as a vector in which case the resolution doesn't matter), at which times you will have to set a proper resolution.
So you don't have to set the resolution on your image, if you export to bitmap in a different format than SVG you will have a field where you can set your resolution on the export dialog. If you print, you'll have to set the resolution on your printer (you'll probably find some option for it in the print dialog). If you are using bitmaps in your SVG file, make sure they are at the proper resolution before importing them into Inkscape.1
Isso também pode ser feito criando uma nova imagem SVG. Como as imagens SVG e seus comportamentos são definidos em arquivos de texto XML, a criação de uma nova imagem SVG eliminará os artefatos de baixa resolução que foram observados em sua imagem SVG anterior de 25x25 milímetros.
1 A resposta de Rico para Como faço para definir resolução no Inkscape?