Assuming you mean this one:
OddWatt 6SL7 SRPP / KT77 Push-Pull Class-A Ultra-Linear DIY Tube Amplifier
Reading the description, built per the schema you're looking at 25 wpc.
As far as the output transformer goes, Bruce is using a 25W 8K primary Edcor with two sets of output taps, 16R and 8R.
Important note on the schematic: He is connecting 8 ohm speakers to the 16 ohm taps, or 4 ohm speakers to the 8 ohm taps. This is effectively halving the load that the output tube see from 8K (the transformer spec) to 4K. You could also use a 5K or possibly even a 3K output transformer it you want. The 5K will reduce output power & distortion a little bit, and the 3K will increase them a bit.
You may want to consider output transformers a little larger-say 35W; more iron gives a little more bass headroom.
The amp is configured in Ultralinear (UL) so your transformers will need screen grid taps (this shouldn't be a problem). You can also configure the outputs to run in triode if you want....
What is your budget? Two obvious choices for output transformers are Edcor and Hammond (esp since you are in Canada). Edcors are great bang for the buck, however, their lead time is commonly 4-6 weeks. They have 3 series to choose from from $ to $$$. The XPP, GXPP, and CXPP. Bruce is spec'ing the CXPP, and after a quick glance at Edcor's product offering, there is possibly one XPP transformer suitable (25W/7.6K) using the speaker tap trick mentioned above. There should be lots of choices in the CXPP series.
Seems like a straightforward project to me, although IIRC the SRPP 6SL7 front end is a bit out of fashion around here presently. The 6SL7 has lots of voltage gain at the expense of current drive.
As far as the power transformer goes, Bruce is spec'ing the Edcor XPR101, so there is an obvious choice. You can use other power transformers if you want; you'll need to tally up the current draw of the tubes, and use Duncan Amps free PSUDII software to help you model the PS to get the correct B+ voltages. Getting a B+ of 350V out of a 240V secondary seems like a little stretch to me, but the secondary of the XPWR101 is rated at 2A!!
I would do the PS a little differently, but that's just me.......You certainly don't need 2A (2000 ma) secondary current, and you also won't be using the 60V bias tap, so there are other good power transformer choices for less than $103 for the XPWR101. I would also use a choke in place of the 25R/5W resistor in the PS.