I'm not sure where you got the idea that larger tanks size would preclude trimming as often? I would still be trimming a larger tank just as often only difference is that it would take longer to trim each time because there's more plants to trim.
Also larger tanks usually have more room for me to correct mistakes/problems that occur, so in that instance bigger is better IMHO.
A bigger tank means more plants. More plants mean that you would need to replace them more if they aren't doing well for any reasons. Don't forget that at any given point, you would want to rescape your tank. I have a 40g and so far have rescaped the tank 5 times over the course of 1 year.
You don't know how the tank should look. You are new. You always think that you are doing everything right and the plants aren't doing well because there are something wrong with the plants. Well, it turns out you aren't doing everything right but you just refuse to see it. We all have been there. A smaller beginner tank is better. It would cost you less to replace or to restock your plants. It is easier to rescape. Changes can make more efficiently. With a big tank, even if your water parameters are out of wack, your plants won't just die like your livestocks. It would take days to see the effect (fert deficiency and melting and etc).
More importantly, you don't have much experiences with various plants and how they would grow in your water and tank conditions. Could you imagine that you have just planted over $100 of vals in your 125g because you thought the plant would look nice. And you start adding Excel because you thought it was a good idea. Can one say melt city???