so technically you can use the same logic to calculate width of day
//oControl is a sap.me.Calendar
var aWeekDays = oControl.getDays();
var iWeekDays = aWeekDays.length;
var iWeeksPerRow = oControl.getWeeksPerRow();
var iDaysInRow = iWeeksPerRow * iWeekDays;
var iDayWidth = (100 / iDaysInRow);
//oRm.addStyle("width", iDayWidth + "%");
that calendar is in S3.view.xml so you would need cretea an extension to modify it