Sorry if this has been asked before, i did search but found nothing. Can the a14 pistons be used in the a15 or would i be better to go down the mazda e5 piston route?

you would be better going down the CA18 route lol na i think you will find they are a bigger dia piston and im not to sure about the pin height. ill call a mate (jeff taylor JTS) he'll know.

No they are not interchangeble. They are the same diameter but have a different pin height. The A15 has a larger crank stroke, while the piston bore diameter is the same as A14.

The A15 piston has a 30mm pin to crown height, and the A14 has a 32.5mm height. The mazda E5 is 29.1mm, hence the need to deck the block. Although the mazda pistons have a much smaller dish, despite the lower crown height. Also the standard diameter of the E5 piston is 77mm, so you'd need to bore your block 1mm over right from the get go. As well as the conrods needing to have the small end opened up to 20mm (from 19mm)to take the mazda gudgeon pin.
Perhaps price up new pistons before deciding which way to go? E5's might be false economy by the time you have your rods modified? Although you will get a higher CR..........

How large can the cylinders get before there are issues? I can get flat top 060 a15 pistons pretty cheap, should i go with them? that would save two lots of machining (conrods and block)

"060" pistons are 0.060 inch oversize (60 thousands of an inch) or 1.524 mm

A15 bore: 76.0 mm
060+bore: 77.5 mm

Nissan sold A15 pistons in Std, 0.5 and 1.0 mm oversizes.
0.5+bore: 76.5 mm
1.0+bore: 77.0 mm

For 1mm oversize, Nissan recommended no sonic testing. But for 1.5mm oversize, maybe you should.

I've out to 2mm oversize and I never tested the block.
Nissan's casting processes were probably somwhat better than the old V8's were. I doubt core shift would be as big an issue with the beloved "A"s.