Ridgid is really the only way to go especially over long runs. We usually spec 4" sched. 20 PVC pipe as it comes in longer lengths than 4" metal does around here , though elbows can be tough to source sometimes. Unfortunately most contractors / HVAC guys will use flexible corrugated hose because it's cheap and fast. If it's a luxury project they'll splurge for the flex aluminum instead of the plastic kind. This type of pipe can cut a fan' effectiveness in half over long runs. Little thought is really given to these fans in most homes today. They are often undersized for a given bathroom and poorly located too. The crappy pipe detailing makes a bad situation worse. Panasonic's website has (or used to) a pretty good tutorial on how to properly size a fan, where to locate it and how to properly pipe it to the outside. I think much of the problem is cost and ignorance. Most people know you get a fan in your bath , but when pressed they won't really be able to tell you its there to expel warm moist air from your home, not smells. They assume that the $34.95 version will be fine for them and a quick & dirty install will get ya out the door for a hundred bucks. The reality is, many master baths, and some hard working kids baths need a much larger fan than 35 bucks can buy. We are often talking $100-$200 for an adequate model, more if you want features or quietness. Add the increased piping cost and labor for the proper venting needed on higher capacity fans and this can quickly become a $500 line item - which starts to look crazy for something as pedestrian looking as a bath fan. Timer switches are also a very good idea as most fans even properly sized ones need to run a while after you've left the bath to completely exhaust the moist air.

A bathroom exhaust fan should produce 4 air exchanges per hour. Your bathroom is 7 x 7 which is 49sq.ft Assuming an 8' ceiling the total volume would be 49 x 8 = 392cu.ft We want four complete air exchanges per hour so that would be 392cu.ft x 4 = 1568cu.ft per hour. There are 60 minutes in an hour so we divide 1568 by 60min and it equals 26.13cfm. You currently have a 50cfm fan, which is twice the size you really require yet your having problems with the mirror fogging up. I suspect your problem is not the fan. Nature abhors a vacuum therefore a fan cannot push air out of a room unless there is provision for an equal amount of air to enter the room. You can test the theory by running the shower and leaving the bathroom door slightly ajar to allow more air to enter the room. Generally they make a slightly higer gap under the bottom of a bathroom door to allow more space for makeup air to enter the room but in some circumstances that does not provide enough makeup air. The solution then is to install a duct through the wall to an adjacent room and put a grille on either side. That will allow the additional air.

Just get a fan that is a little bigger than the fan you presently have. the use a drywall saw and razor knife to cut the opening bigger. Just make sure you get the size in CFM for the size of your bathroom. Fans are rated in cubic feet per minute and sones. CFM can be calculated using the link below. Sones are the amount of noise they make. the lower the sone # the quieter the fan. A rule of thumb the more expensive the fan the quieter it is. Here is a link that might be useful: calculating the size of bathroom fan.

I just bought 2 nutone heat, vent light fans for $195 each at Lowes. nice product, 2 sones (very quite) with direction adjustable heat vent in cover. about 2" larger than existing bath fans. perfect imo, to just cut the hole a bit larger. I also installed daylight cfl bulbs. used hardcast brand mastic tape to seal housing to cut in sheetrock before installing cover. also used the mastic tape to install the damper (and penny) to the housing of the bath fan, and the venting to the damper. took about an hour per bath fan. not my first rodeo. I also like panasonic whisper quite fans, but twice the price of the nutone. never liked nutone before, but saw mostly elcheepo bath fans that sounded like a 747 taking off. these were quite nice, and the low sones rating means that they will be used. I recommend running the fan for half an hour after shower. best of luck.