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Maybe I missed something so simple here. But I want to calculate the max size of a font given a rect. My solution is below. Is crap. I am sure there is a better way. I did a little extreme in the loop.

But a more streamlined func or a built in way to calculate the font_size most welcomed.

# coding: utf-8
import ui

# only for testing...
_inset = 0.0
_font_name = 'Avenir Next Condensed'
_frame = ui.Rect(0,0, 320,480)

def font_size_for_rect(rect, text, font_name,
                            inset_factor = 0.0):
        fs_size = 0

        # inset the rect, by a factor for width and height
        # the inset can be very helpful
        inset_rect = ui.Rect(rect.width * inset_factor,
                                        rect.height * inset_factor  )

        r = ui.Rect(*rect).inset(*inset_rect)

        for fs in xrange(0 , 1000):
            fw, fh = ui.measure_string(text, max_width=0, font=(font_name, fs), alignment=ui.ALIGN_LEFT, line_break_mode=ui.LB_TRUNCATE_TAIL)

            if fw > r.width or fh > r.height:
                fs_size = fs -1
                break
        return fs_size

if __name__ == '__main__':
    _frame = ui.Rect(0,0, 320, 480)
    v = ui.View(frame = _frame )
    btn = ui.Button(frame = _frame)
    btn.title = '99'
    btn.bg_color = 'purple'

    fs = font_size_for_rect(_frame, '99', _font_name,
                            inset_factor = _inset)

    print 'Calculated Font Size = ' ,  fs
    btn.font = (_font_name, fs)
    v.add_subview(btn)
    v.present('sheet')

doesn't a given text scale linearly with font size? If so, you could measure string with a nominal size, then simply compute the ratio needed to just fill the closest dimension, and apply that to the font size.