Does the following work ?
UIFont * customFont = [UIFont fontWithName:ProximaNovaSemibold size:12]; //custom font
NSString * text = [self fromSender];
CGSize labelSize = [text sizeWithFont:customFont constrainedToSize:CGSizeMake(380, 20) lineBreakMode:NSLineBreakByTruncatingTail];
UILabel *fromLabel = [[UILabel alloc]initWithFrame:CGRectMake(91, 15, labelSize.width, labelSize.height)];
fromLabel.text = text;
fromLabel.font = customFont;
fromLabel.numberOfLines = 1;
fromLabel.baselineAdjustment = UIBaselineAdjustmentAlignBaselines; // or UIBaselineAdjustmentAlignCenters, or UIBaselineAdjustmentNone
fromLabel.adjustsFontSizeToFitWidth = YES;
fromLabel.adjustsLetterSpacingToFitWidth = YES;
fromLabel.minimumScaleFactor = 10.0f/12.0f;
fromLabel.clipsToBounds = YES;
fromLabel.backgroundColor = [UIColor clearColor];
fromLabel.textColor = [UIColor blackColor];
fromLabel.textAlignment = NSTextAlignmentLeft;
[collapsedViewContainer addSubview:fromLabel];
edit : I believe you may encounter a problem using both adjustsFontSizeToFitWidth and minimumScaleFactor. The former states that you also needs to set a minimumFontWidth (otherwhise it may shrink to something quite unreadable according to my test), but this is deprecated and replaced by the later.
edit 2 : Nevermind, outdated documentation. adjustsFontSizeToFitWidth needs minimumScaleFactor, just be sure no to pass it 0 as a minimumScaleFactor (integer division, 10/12 return 0).
Small change on the baselineAdjustment value too.