As Deribus mentioned, I think reducing the size without changing the stats would make it too powerful in firebases, since you can basically stack 4+ instead of 3 shields to defend a T2 PD and they are high HP and low cost compared to UEF/Cybran.
Shield HP Mass HP/Mass Cybran ED2 7000 460 15.22 Cybran T2 ED1 4000 160 25.00 Sera T2 13000 700 18.57 UEF T2 9000 600 15.00 Aeon T2 11000 480 22.92Here's the overlap possible for a PD with the current shields:
5470f661-39dd-449f-9674-bfd7c851af38-{F9516D52-138F-4B4C-A8CF-E9D47783D4F8}.png
I don't think being unable to upgrade the shield is a huge drawback, it just requires you to ctrl-k the shield and reclaim the small wreck before building the very cheap T3 shield. It requires some APM but definitely isn't impossible if you're defending something important that requires your attention with shields.
For the size complaint, I'm not sure what you want to shield. It can shield adjacent structures pretty well. Not entirely but almost everything:
fa870dbf-585d-4f6f-8b34-a5ca92cbf4bc-{790AA499-AC52-4949-9C23-C4EFCD199CFF}.png
For this pgen specific case I'd rather opt for a tiny increase in shield size instead of making the structure smaller.
I've been testing your size idea with the "Create entities" dialog (its in the hotkeys, you use the hotkey while a unit is selected and you can change its blueprint values. Spawn a new copy of the unit to make sure you get all the changes. Requires cheats):
4x4 size is just way too OP, since you get ~8 overlapping shields instead of 3:
0f8146df-8744-4fd2-8b93-37ffeee1a32b-{B23F00F0-CCDE-4267-8FBD-7183F0AFB807}.png
5x5 size is enough to cover a pgen but its still a small shield so its not reaching towards diagonals:
c37ebfaf-9937-4a09-ae23-c5274924572a-{1C4F88FF-55A4-42B5-829D-7077D634F129}.png
It also still increases the coverage from 3 to 4-6 shields:
e517f83b-d566-4420-a13e-697eca9fd3fb-{964C95FB-5B81-4CE1-A786-2607A7EEBCFA}.png
Decreasing the size even by 1 seems to bring more problems than solutions.
All in all, I understand why you dislike the shield but I think its strengths in HP and cost make it above average for firebases and protecting most directly adjacent things like T3 shields, pgens, factories, or engineers. The T3 shield is also very strong because it's super cheap and the 2nd most powerful, while giving sufficient radius imo: I've seen Aeon T3 shields easily protect air grids, artillery, or game enders from artillery barrages when a sufficient number are built (and number of shields is the most mass-efficient way of defending, so having a cheap shield helps a huge amount - natural regen is nearly worthless while recharge from 0 is extremely strong).
Shield Shield HP Mass BT HP/Mass Sera T3 21000 3600 5841 5.83 UEF T3 17000 3300 4988 5.15 Aeon T3 18000 2400 4097 7.50 Cybran ED5 16500 4260 7100 3.87 Cybran T3 ED4 13000 2460 3515 5.28