So I know that with the 2.0 rulebook the size restrictions are rather vague. The old rules had the max length of a side to be 36 inches and there was a maximum surface area but I can't remember/find what it was. I'm looking to make either a roman scutum (tower shield) or a proper greek Aegis (they're rather large) what are some thoughts, general rules of construction, what I should look out for, etc.

The current tower shield I'm designing is 48 in. tall and 23ish in. wide (which will be bent to much less)
Would this work? How do logistics marshals feel about this?
In the 2.0 rules, there is no maximum or minimum size for a shield; as a general rule, so long as it's more or less Shield-like and you can physically hold and carry it, you're probably allowed to bring it into game (providing it passes padding/safety checks, of course).

The Shield you're describing sounds like it should be fine, although heavy and tiring to carry during extended combat if the main 'body' is made out of something heavy like wood; try to go for lighter materials like plastic or foam when constructing it.

Also check with your local chapter to see if anyone can help you make it; most chapters have at least one person who makes or has made weapons and shields for themselves or others, and in many cases they'll be willing to share tips or otherwise assist you with construction.

As a side note, if your shield is going to be that big, I'm going to highly recommend you practice carrying it around for a couple of hours at home to see whether you like it or not, and to also consider making a similar but smaller shield in case the large one proves too heavy or cumbersome for your liking/comfort.