在Draw.annotate方法中，width参数似乎对呈现文本没有影响。
我也遇到过同样的问题，我开发了这个函数，通过添加新行来使文本适合指定的宽度。
我有一个功能来检查一个文本是否适合一个指定的宽度时，画在图像上

def text_fit?(text, width)
  tmp_image = Image.new(width, 500)
  drawing = Draw.new
  drawing.annotate(tmp_image, 0, 0, 0, 0, text) { |txt|
    txt.gravity = Magick::NorthGravity
    txt.pointsize = 22
    txt.fill = "#ffffff"
    txt.font_family = 'helvetica'
    txt.font_weight = Magick::BoldWeight
  }
  metrics = drawing.get_multiline_type_metrics(tmp_image, text)
  (metrics.width < width)
end

我还有另一个函数，通过添加新行来转换文本以适合指定的宽度

def fit_text(text, width)
  separator = ' '
  line = ''

  if not text_fit?(text, width) and text.include? separator
    i = 0
    text.split(separator).each do |word|
      if i == 0
        tmp_line = line + word
      else
        tmp_line = line + separator + word
      end

      if text_fit?(tmp_line, width)
        unless i == 0
          line += separator
        end
        line += word
      else
        unless i == 0
          line +=  '\n'
        end
        line += word
      end
      i += 1
    end
    text = line
  end
  text
end

试试看：

phrase = "my super long text that needs to be auto line breaked and cropped"
background = Magick::Image.read('my_bg_img.jpg').first
image = Magick::Image.read("caption:#{phrase}") do
  self.size = '800x600' #Text box size
  self.background_color = 'none' #transparent
  self.pointsize = 22 # font size
  self.font = 'Helvetica' #font family
  self.fill = 'gray' #font color
  self.gravity = Magick::CenterGravity #Text orientation
end.first
background.composite!(image, Magick::NorthEastGravity, 20, 40, Magick::OverCompositeOp)
background.format = "jpeg"
return background.to_blob

找到了一些代码here，可以做到这一点：）