Hi NickD
Fair question and I should really remember that my past ventures into cars and planes has perhaps led to me expecting all modellers to understand what may have been generally understood in the past. I refer to a typical standard servo as not being micro or mini and not digital resulting in prices ranging from about £6 up to £15 depending on the gearing. You can get cheap digital but they will use more current and are really more suited for the flyboys where really accurate positioning is required.
I suggest you visit E-Bay and type in Standard Servo to see a range of options. There are many on-line suppliers and all will help you with your particular requirements and can be contacted by phone.
You may be aware that some Chinese imports may be of questionable quality and I have a drawful of such items that my fellow club members have had fail. I like repairing where possible so the bits come in useful.
Finally I should mention that some micro and mini servos has a max voltage of 5v and will be damaged by any voltage above this level. Personal experience shows that they will melt so beware.
Hope you can find the servo you need
Dave976

Regarding Standard Servo sizes, there are Standard size Servos, similar to the Futaba/Robbe S33, and there are the mini(micro servo) which are less than half the size of Standard, used in tight quarters.
I can post pics if needed.

Dave, in your opinion what's a standard size servo please.

Hi NickD
Personally I have never had a problem with a standard sized servo controlling a rudder, regardless of the rudder size. That said the force required will increase if you are using a long servo arm at the outer holes. Ideally the servo arm should be the same size as the arm on the rudder. All servos will deliver their maximum force using the holes nearest to the pivot point. Pre programmable radio the rudder throw was adjusted by moving the connecting rod to different holes, reducing the force available as a result. With modern radios you can adjust the servo throw to suit your needs without any loss of force.
As others have said free movement of the rudder and linkages, using a balanced rudder and not requiring much throw at speed all have an effect. Metal gears are more robust and may prove more resilient to the odd accident and many servos now come with such as standard. On my faster models I use a switch to have dual rates on my rudder(s) so I don't capsize then when turning at speed (it's an age related problem), but can have maximum control when sailing slowly. Hope this helps you make the best choice.
dave976

As others have posted, if you have a semi balanced rudder with 25% in front of the rudder post, then a regular High Torque metal gear servo will work quite fine.
Many of my R/C boats have 3.5" tall X 2.25" wide rudders, which work very well with decent, full size servos
See photos

Hi if you 'balance' the rudder say one third of the area in front of the rudder stock. Then a slightly better servo say 5kgrm will do the job.
I suspect you will not need much rudder area either as the slightest change to the waterflow under power will have a big effect.
Roy

It depends on size and shape of rudder, also the ease of movement.
But if you have servos available then use the strongest, that way you shouldn't have any problems.
Cheers Colin.