Setting fixed width of columns in IR in APEX 5 is a little different than in older APEX version. Here's an simple example of setting fixed width of Hiredate column of Employees report.
First you have to set column static ID:
If you have more than one report on the page you can prefix CSS with region static ID, for example:
Update:
Use min-width CSS property instead of width (it fixes problem if you have many columns in report)
#regionStaticId th#colHiredate, #regionStaticId td[headers=colHiredate]{min-width:100px}
You can check out demo here.