As a rule of thumb you always want your filter to have a higher CFM rating then your fan.... the slower the air moves through the filter the better it filters...So if your fan is rated to 105 CFM get a filter rated for 150 cfm or bigger,, this way you can run your fan at full speed and it will , one let air pass through your fan with less restriction and two it will filter better aswell... I use a 8" fan for my 1000w with a speed fan controller......Its always better to get a bigger fan then you need for many reasons...... One reason is that with a bigger fan you can move the same amount of air as a smaller fan but it will be less noisey.. EX. My 4" fan running at full speed is about 150 CFM and if i take my 8" fan that runs full speed at 600 cfm but turn it down to 150 it will be less nosiey then the 4" at full speed even though they both are pushing the same amount of CFM.....MAKE SENSE??
Another reason is that i can run my fan slow when its cold and i can turn it up to 600cfm if it gets hot.... This allows me to adjust my fan for the envierment. but if you had the 4" you are pretty much stuck just using it full blast all the time and if that ant enough then you are screwed...... So haveing a bigger fan then what you need is usually a good idea for the reasons i just stated above...