CATextlayer change size of FONT to fit the frame

I have a CATextlayer of a certain size and NSAttributedString text of unknown length.

I need to adjust the font-size so the text fits the frame (not vice versa :)

Any ideas where to start? :)

[Edit] as nall points out, I can determine the string length, of course, it's some text entered by the user that I need to fit into a box of fixed size.

I ended up doing this:

textlayer is a CATextlayer

theString is a NSMutableAttributedString

And yes, it's not very elegant and could definitely be improved ;)

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)theString);

    CGRect columnRect = CGRectMake(0, 0 , 320, 150);

    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddRect(path, NULL, columnRect);

    CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, NULL);

    CFRange frameRange = CTFrameGetVisibleStringRange(frame); 

    int fontSize = 18;

    while(theString.string.length > frameRange.length){

        fontSize--;

        CFStringRef fontName = (__bridge CFStringRef)[defs objectForKey:@"font"];

        CTFontRef font = CTFontCreateWithName(fontName, fontSize, NULL);

        [theString addAttribute:(NSString *)kCTFontAttributeName
                          value:(__bridge id)font
                          range:NSMakeRange(0, theString.string.length)];

        CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((__bridge CFAttributedStringRef)theString);

        CGRect columnRect = CGRectMake(0, 0 , 320, 150);

        CGMutablePathRef path = CGPathCreateMutable();
        CGPathAddRect(path, NULL, columnRect);

        CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, 0), path, NULL);

        frameRange = CTFrameGetVisibleStringRange(frame); 

        textLayer.string = theString;
    }