There are a few tubes that are available. The MAF is one of the last restrictive items on the motor that usually its only benefits of a larger tube is when you are matching it to the charge piping from a FI set up. At that point you have enough air flowing that you also need the Diablo MAF.
Please correct me if I am wrong but you will be in the noise (no measurable gain) by going with a larger tube on a light to medium NA modified motor.
As a general rule, a large MAF tube is only used to increase the upper resolution of the MAF.
IIRC the stock MAF tops out around 20 #/min, or about 200 HP.
When you go larger you 'sample' less percentage of the air and that allows you to get more air before the MAF taps out.
Problem with a larger MAF is you do loose resolution on the low end. The air is moving slower and accuracy can suffer some. But a good tuner can move things around to get it to work well in the most common areas you'll drive in. There's actually quite a bit a tuner can change to make it work very well and accurate, if they want to spend the time.
In a nutshell, unless you go boost,,, you won't need a larger one.
If you really want a larger one just go with a stock SVT MAF and the stock SVT injectors. The SVT injectors are matched to the MAF and will work untuned. The SVT MAF is 69mm a 70mm MAF is 3". So you could do a stock upgrade and keep it untuned but really it would be a mute point since you would really need supporting mods as just upgrading the MAF will do nothing. No point in getting more air in if you can't get it out and you'll get more power from the SVT header than most other mods.
Actually you cannot do that change untuned , altho the injectors do match the MAF electronics/ MAF tube size the fueling in the stock tunes between the SVT and the Zetec are not the same and fueling would be off