Starting Attributes and SIZE does matter

The highest starting stat I've seen for a slave at the slave market is 100, I don't know if it can go higher but it stands to reason that may be the cap for a starting gladiator. Each attribute is important, but some are more important than others depending on your specialty and build. For example many of the skills in the Theatrics tree benefit from having a high presence.

Your size attribute is very important. Aside from being the chief attribute when determining your health, it is the most important one when determining how much gear you can carry without being encumbered. Strength plays a roll in this, however if you have an small to average size it is going to be tough to wear the strongest and heaviest armor.

Size is also the only attribute you can not train, so be sure the slave you buy has a suitable size for whatever build you are going for. If you are going for a tank or someone who will take a lot of damage, then get a high size.

Size is one of those things where you can't have too much, but you can have too little, so plan accordingly and if you find yourself with a gladiator who's size isn't what it should be for the build you are going for your options are to sell him and buy a new slave or go with a different build.