I recently acquired some brush pens meant for calligraphy, one of size 'standard', and one of size 'beginner'. While I cannot find the site where they were labeled as such, the lineup is here with XFL2U being the 'standard' and XFL2F being the 'beginner'.
Because of its availability where I live, I've been practicing on 5x5mm graph paper, writing 10x10mm kanji with a pen. I've seen proper kanji practice paper with 8x8mm boxes.
I can make both brush pens work in 10x10mm, but it gets very finicky with more busy kanji, like 慶 or 鬱. Moreover, I feel like I'm utilizing very little of the range of line width, and I have too little to work with to properly take advantage of the brush, to have the motion of the strokes reflected in the character rather than merely meticulously drawing a figure.
I get that this is an art that takes practice, but the 'standard' brush in particular feels too big for an 8x8mm square (which I assume is for pen/pencil), because so little of its range is used. I assume a 'standard' brush dictates a certain 'standard' line width range, which in turn dictates a certain size of kanji determined by their 'maximum busyness'. A minimum practical size follows the brush.
So, if a Japanese person shows up for a caligraphy course, is given a standard brush and a box to draw within, how big is the box?