My 3x8 had a rear cleanout door to help get the flues clean, 3' will mean you would need a sectional handle, I had to do that and it was a chore and I had 4 feet between the rear of the evaporator, thus 2 sections. It would have been much better with more room at the rear of the arch. cleaning flues from the front is much harder because of the distance before you get to the flue pan. I suggest you visit some sugargouses with 10' long arches, and see what they suggest. The sides, just plan out what equipment will be there and how much extra space does it need.
My sugarhouse is only 16x24 and I planned the evaporator footing for a 3x8. What I didn't realize was how much room would be needed for some things. AS I enter the sugarhouse man door, on the left I have 6' of built in shelves, then a chest freezer, a chest type fridge, 6' of cpunter with a 225 A electric panel on the wall over the counter then 6' of RO room. Then across the back it's 2 double hinged doors. Then up the other side is my filter press, then my water jacketed bottler, then a 2x6 finisher (which because of the tube burners fills almost 8' length, then a counter to the corner. Over this counter is a large inverter for solar, and an electric panel for the solar portion, battery backup. The battery bank is mounted on the outside wall. Then on the last wall leading to the man door just before the corner is all counter and sink with a tankless water heater over the sink. finally there is about 28" open, my RO is on 4 wheels and when using it, I roll it and park it in that space. To put it mildly I failed to realize how much space I needed, While I was running a wood fired 3x8, I was able to keep the space around the evaporator passable, it's quite crowded. Also over the counter to the right of my sink is where my solar comes into the sugarhouse. There I have an 80A charge controller, A solar breaker box, a electric meter recording the KWH generated by the grid tied portion of my solar. I have a dual system, 4840 watts of grid tied only (But I can use as much as I need, unless the grid goes down, which in 13 yrs so far I only had one time of about 30-45 minutes when it went out. My battery back-up solar is 1480 watts, it charges the battery bank and then sends all excess power to the grid, if the grid goes down it instantly runs off the battery bank.
Also, next to the double doors outside the sugarhouse I have a small bulk propane tank. Off that I have about 14' of 3/4" black pupe for propane with take offs for an old propane fired bottler, (now gone) and a weed burner torch, used for lighting the evaporator, at the far end of the finisher I have 3 propane take-offs, one goes up into the trusses , across to the font corner, down and it fires a small no electric needed propane wall furnace. From about the same spot where that propane furnace line branches off the 3/4" main, I have a valve fueling the 2x6 finisher and a valve fueling the tankless wather heater and 1 spare valve, just in case.
As you can see I should have built my sugarhouse 20x30 or even 24x36, but back when I was burning a 2x6 wood fired evaporator and didn't have all of the extras it seemed plenty large.
All I'm suggesting is that you plan better than I did. Over the 23 yrs since I built the sugarhouse I've drawn out a few sets of plans for a kitched addition, but it never happened. Now at 78 I've cut back to a 2x6, converting it to oil fired and I just can't see an addition ever happening at this point, it's easier to build it right the first time rather than trying to build addition after addition as more room is needed.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.