When targeting a figure that takes up 2 or more squares do you have to choose the square you are targeting so that effects like Blast cause damage to figures and objects adjacent to that square, or when you target an E-Web and cause damage enough to activate Blast does it damage all figures and objects adjacent to the E-Web thus making it up to 10 squares around instead of the 8 squares it would be if you had to target a specific square.

You only take the squares adjacent to the square you were targeting. See also Blast in the RRG:

If the target is a large figure, Blast only affects figures adjacent to the targeted space and does not affect the target figure.

Hmmm why is that? Its like shooting a bazooka at a tank and all it does is hurt the troops around and no damage to the tank. I don't see the logic?

The tank in your example already suffered the damage from the initial attack. Why should it damage it twice and as this was a rule question, the answer stands in the rules.

True it should not be damaged twice, but neither is a single figure.. it's only the adjacent figure that gets additional damage, so pointing out specifically that a large figure does not get damage makes it a bit confusing and would even suggest the tank gets no damage (which is not possible as it has to have damage to trigger the blast)

so hence my question, maybe I missed something in the rules or interpretation of them as the large figure is mentioned specifically.

The figure takes damage if it is adjacent to the targeted square

for example, where T is target, E is Eweb, a is at st and S is stormtrooper

_SEE_ S

__ET_

____AAA

____AAA

____AAA

If you target square T of the Eweb, the Eweb would take damage from teh attack, and then Blast would affect the AT-ST since it is adjacent to the target square, but the stromtroopers would not, as neither are adjacent to that square.

Think of it as blast targeting only the square itself, not the unit it contains

Think of it this way:

A Blast is only 9 squares big, regardless if you are hitting a Trooper or a Star Destroyer. The blast doesn't get bigger just because you hit something bigger.