In my search to answer that same question for myself I found some interesting info, first is this thread which shows the calculations for stress on a splitter beam:
New splitter ideas
The "section modulus" is the value we are looking for.
Here is a chart that shows wide flange or "H" beams:
American Wide Flange Beams
Here is a chart that shows rectangle beams:
Rectangular tubing chart
I chose rectangle beam because a rectangle with its narrow side up is stronger than a square beam of equal size but this rectangle tube calculator works for square tube too.
Engineering Section Properties: Rectangle Tube Calculator
All this assumes that the section modulus for a H beam and a rectangle tube are the same. They should be unless a tube is more/less resistant to bending force than a H beam is. My metal fabricator friend says tube is better for splitters than H beam and he's been doing metal fab his whole life.
Here is a hydraulic cylinder force calculator:
Hydraulic force - area formulas and calculator
Since a 5" ram is more common than a 4.5" one we'll assume you go with a 5" and also assuming a 12" tall wedge your section modulus to beat is 33.3 cubic inches.
the section modulus on a 5X5X1/4 tube is 7.16458, not even close to good enough.
Here are some beams to consider:
10X12X1/4" tube = 39.32 modulus 36.03 lbs/ft
8X10X3/8" tube = 37.69 modulus 42.79 lbs/ft
8X10X1/2" tube = 48.28 modulus 55.66 lbs/ft
6X8X5/8" tube = 33.56 modulus 51.80 lbs/ft
W8X40lbs beam = 35.5 modulus 40.00 lbs/ft
Now if you want to go with a 4X24 cylinder at 3000 psi you would have 37680 lbs of force against a 12" wedge which gives you a section modulus of 20.93 cubic inches so your 5X5 would still be way too weak.
As for your 11.5 hp engine, it is fully capable of running a 16 gpm 2 stage pump (8 HP min.) but a 12 hp engine is recommended for 22 gpm 2 stage pump.
