Psu change

If the PSU sits Outside the Case after removing the Internal PSU and uses Moddiy Adapter. Only Short cards Fit at all.  10 to 12 inch cards will not physically fit inside the case.

https://www.moddiy.com/products/Dell-OptiPlex-3040-3050-3060-Main-Power-24-Pin-to-6-Pin-Adapter-Cable.html

Small units with large cards will also need PCI-E riser cable and the cover open.

https://www.amazon.com/Thermaltake-AC-045-CN1OTN-C1-Premium-Extender-Graphic/dp/B06Y1SX7ZN

NON Dell cards will require Secure boot OFF.

EVGA 700BR is bare minimum recommended for RTX 3060 in a Dell.

https://www.evga.com/products/product.aspx?pn=100-BR-0700-K1

"What are the dimensions? "

Really Really Small

There is not a lot of detail on the spec sheet.

Physically and Pinout it will not take a standard unit because its Proprietary. 

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/OptiPlex_3060_Spec_Sheet.pdf

(MT)   13.8” x 6.1” x 10.8” / 35 x 15.4 x 27.4
(SFF)  11.4” x 3.7” x 11.5” / 29.0 x 9.26 x 29.2
(Micro)  7.2” x 1.4” x 7.0” / 18.2 x 3.6 x 17.8

SFX has dimensions of

125 × 100 × 63.5 mm (width × depth × height)

I could really use dimensions, they don't have a spec sheet for it. 

The card is 242 mm long, and I'm most probs using cx750f. I want the psu to be inside the case. 

Not sure how you fit CX750F inside ANY 3060 due to the amount of room available in the case.

The Moddiy Adapter is NOT optional.

SFX dimensions

125 × 100 × 63.5 mm (width × depth × height)

ATX CX750F has dimensions

150 × 140 × 86  mm (width × depth × height)

https://www.moddiy.com/products/Dell-OptiPlex-3040-3050-3060-Main-Power-24-Pin-to-6-Pin-Adapter-Cable.html

https://www.corsair.com/ww/en/Categories/Products/CORSAIR-iCUE/Power-Supplies/iCUE-CXF-RGB-Power-Supplies/p/CP-9020218-IN

So it can't fit? Whats a good psu that can? 

• Thank you. Looks like cx750f will fit, it's just 3.5 cm more wide than SFF. Thank you so much I appreciate it. 3060 ti will fit in case right. 

There are no standard units of ANY size that fit at all.

It may be possible to fit an SFX unit inside HOWEVER it sits in the bottom, must be upside down , has too little room for air flow and requires quite a lot of modification if its going to be permanent. YMMV

Power supply needs to provide 140W -  225W combined on the 3.3v/5v rails and again the Moddiy adapter is not optional.

The other way people did this was to take all the parts and mount them on a board without a case. 

The other reason to talk about SFX is that it allows you to use the bracket as a template should you attempt to modify the case to fit the PSU upside down in the case. There are no tall feet and no vents on the bottom of the dell. So you either make holes OR you flip the PSU around to allow air into the psu.  Otherwise it will FRY and Die due to overheating.  You also have to cut a large hole in the back using the Bracket as a template so the PSU can vent air out the back.

Of course i even been able to put a 600 watt psu inside mine

 with some help of a zip cut it sitted perfectly