I'm a bit of a planner snob, so I've tried out many of the options available. In my opinion (for my use), you'll want something large enough to handle all of your daily tasks, but also not so large that it will be cumbersome, making you less likely to use it. For my first two years of undergrad, I used the "At-A-Glance" brand (I believe mine came from Staples), because they are about the size of notebooks and have plenty of space for my activities, meetings, lab work, classwork, etc. For my last year of undergrad, I took the plunge and purchased an Erin Condren Life Planner, which, in all its glory, is an excellent planner, but ended up being overkill for me. I enjoyed that it gave me ample space to write down tasks, but there were also so many aspects of it that I felt were unnecessary. To start off my grad school adventure, I'm going to use a bullet journal, which, from my lurking on youtube, seems to be a great way to customize your planner experience and give you exactly what you're looking for. The catch, though, is that it's just a notebook so there will be some work on your end to set one up the way you please, but it can be as simple or as detailed as you'd like. I'm excited to try this method for keeping my tasks in order, but also for accountability for other things like finances and healthy eating.