I want to renovate my shower/tub area in my bathroom. I want to do use white tiles. What size tiles would be best for a beginner?

For a 70 year old house and for a beginner to tiling I would suggest subway tiles which are typically 3" x 6". They are an easy size to install, go in with very small spacing, are easy to cut for corners and edges and they are appropriate for the era the house was built and are stylish. The 6" x 6" tiles will look rather dated. Rent a good wet tile cutting saw while you do the installation.

6x6" might look like floor tiles installed on your wall. It's a very common size in kitchens, particularly as terra cotta on commercial kitchen floors. Then again I tend to look at 4x4" tiles and think "bathroom." So really what size you use is a highly individual consideration, bearing in mind that if you want to sell the house sometime in the future, a weird size might be offputting to your potential buyers.

Little tiles are just as easy to install as bigger ones and indeed, as you have been told, they will hide wall irregularities well.

In order to make your dollars go farther and to give yourself a chance to take things to a posher level, consider breaking things up a bit:

Enormous tiles can also mask wall irregularities simply by bridging over them. You may want to carefully lay things out so that the worst irregularities land in the grout lines, where they can be neatly hidden. Tricky but not impossible (depending).

Modest tile sizes are boring to look at but very affordable. The savings may open enough room in the budget for a band of feature or specialty tiles, which both alleviates the oppressive, featureless expanse of all-one-size tile work and also breaks up the visual space - which also interrupts the perception of wall irregularities.

If you decide on a light color for your grout, make the decision NOW to seal it. Few things are a bigger pain in the butt than trying to clean grout when it gets skunky. I generally lean very strongly toward dark grout but sometimes there's a job that just insists on white or light colors, and then you want to seal it against possible problems down the road. If possible, use a grout that is already mildew resistant, epoxy based, or both - and then seal it anyway.

Good luck with it.