Why this question?
Recommendations are just recommendations only.
This is depending on the scope in use indeed but not only depending more on your own vision ability during visual observation.
The third step consists in the seeing parameter that degrade the perfect.
Perfect being the couple scope-own eyes, different from a guy to an other with or without experience.
About the above posts something is quite absent this the strehl of the opics in cause, any design.
An aperture, a design of opics and the opical acuracy allow to reach some contrast levels considering the size of the feature observed.
Useless to speak the merit of the apo against the newton, mak, CC, etc... To be sure watch the ftm curves of each, contrasts are better for low frequencies in an apo, for high frequencies absolutly not and contrasts may be washed in presence of average apo optics. Stunning ! but obvious, fpl 51, 53, XX etc nothing to see as main parameter.
And we must on aware with control bulletin with given data sometimes optimistic.
Writing this, for me enoughly evident now, remains the initial question of this forum, access to venusian features.
Venusian features are low contrast level, say 1-2% depending also on the color of observation, but on all the light spectra, features are there.
How it is possible to reach such levels?
1- with high proven strehl optics that not degrade high frequencies,
2- good training with visual observations,
3- find solutions in order to adjust the light level of the disk proper to reveal features when cancelling the light glare,
4- get images 7/10 and better,
5- a recall have a perfect aligned and collimated optic, not approximative, any design.
Now it is said, 3" refractor 6" reflector.
On the 80ies I started on venus with a 3" polarex unitron (you now an achromat making violeted images) and a simple 4.5" newtonian tube fully open. This allow me me to follow on an elongation day to day a correlation between high altitude formations on venus and some white bright spots occuring in the atmosphere.
More recently, still on venus a simple 2" refractor revealed banding structure of the atmosphere, ashen light also but this is an other topic and controversial.
What I can conclude a 2" refractor can do, a little but can do, a 4" reflector can do also, but with high strehl optics.
With such seeing parameters is almost negligeable.
We could speak about uranus also a substantial more difficult target, this is more crucial to get something but with high strehl optics, adequate seeing and good viewing abilities.
I did some tests on the ground in order to verify all these abilities/assessments when viewing long distance targets with very low contrasted banding features with the help of a vc200L, high strehl and 42%CO.
This is changing to the always theorical considerations developped into forums .
Observing planets even an uranus depends on mesopic vision, not photopic, this is a strange field that is variable between observers.
Keep in mind also that the observation is a match between the scope in use and an eye through a filter and an eyepiece, that adjust the light level and that must properly be set to reach an optimum, for reaching the potential resolution limit of the image at the focus plan of your scope.
Always 3 stages to consider, adding the seeing filter, but globally improved by the training and experience of you.
No need to push a 20" for the exercise.