Can every artwork thumbnail be the same size?

Yes. Open Art Studio, Site Settings and set Artwork Thumbnails to Square. Every work on your Artwork page then fills the same square box, so tall, wide and small works line up in one even grid. A picture that is not square is cropped to fit. The crop keeps the center of the picture, or its focal point when it has one. To set it, open the work in Art Studio, Pieces, and click the picture under Images. Whole picture, the default, shows all of each work in its own shape.

Fitted also shows all of each work, with no cropping, and puts every work in one box shape so the grid stays even. The shape is worked out from the works being shown: a list of mostly upright works gets an upright box, a list of mostly wide works a wide one, so the pictures fill their boxes as far as they can. A different collection or decade can take a different shape.

The setting covers the Artwork page in both the Classic and Modern styles. A work's own page always shows the whole picture.

To make the pictures larger, set Thumbnail Size on the same screen to Medium or Large. That covers the Classic style, which shows three works across. In the Modern style, the Columns setting on the page's Artwork Archive block sets the size: fewer columns, larger pictures.