hiii anon!! rentry has a resize function built in the markdown! if you use markdown like '{100px:100px}' it'll resize it for you
it also uses different units instead of px! alternatively, you can use:
- px : 0 - 4000
- % : 0 - 500 (note: percentage height often does not do anything)
- vw : 0 - 500 (percentage of the vertical size of the page)
- hw : 0 - 500 (percentage of the horizontal size of the page)
no units: numbers used as pixels
i often size down based on the ratio of pixels on my graphics (e.g. 1000px by 806px = 350px by 170px) i often rely on my graphics program to do the maths for me regarding sizing down asw

however if ur not using ur graphics/art program to resize then try ur best not to use transform to resize :) it often ruins the quality so i instead try to resize the image + canvas itself
also depends on the size of the graphics ur going for too. i often work on 1000px or above when it comes to main graphics and work it down to about 300-500px. if its for text i usually work below 50px :)
lmk if i helped!