You could just increment the font size until you find a fit. font.getsize() is the function that tells you how large the rendered text is.
from PIL import ImageFont, ImageDraw, Image
image = Image.open('hsvwheel.png')
draw = ImageDraw.Draw(image)
txt = "Hello World"
fontsize = 1 # starting font size
# portion of image width you want text width to be
img_fraction = 0.50
font = ImageFont.truetype("arial.ttf", fontsize)
while font.getsize(txt)[0] < img_fraction*image.size[0]:
# iterate until the text size is just larger than the criteria
fontsize += 1
font = ImageFont.truetype("arial.ttf", fontsize)
# optionally de-increment to be sure it is less than criteria
fontsize -= 1
font = ImageFont.truetype("arial.ttf", fontsize)
print('final font size',fontsize)
draw.text((10, 25), txt, font=font) # put the text on the image
image.save('hsvwheel_txt.png') # save it
If this is not efficient enough for you, you can implement a root-finding scheme, but I’m guessing that the font.getsize() function is small potatoes compared to the rest of your image editing processes.