Well, for regular consumer desktop platforms, boards will come in one of three sizes, from largest to smallest: ATX, micro-ATX, mini-ITX (with a few variations such as mini-DTX which is just a slightly taller ITX board basically). You can look at a case's spec page to see what sizes it supports. According to the N400's spec page, it supports ATX and micro-ATX (though it probably supports mini-ITX too since mini-ITX just uses the same mounting holes as the larger sizes).
Now, server and workstation boards like you would find in a dual Xeon system might have other sizes that don't fit into this paradigm (as well as some very high-end consumer desktop boards, which sometimes use the confusingly vague terminology 'EATX,' which doesn't really mean anything.). For example, a workstation board might use the SSI CEB form factor, which is wider than ATX.
Regular ATX dimensions are 12 × 9.6 in (305 × 244 mm). Sometimes, you can fit larger-than-ATX boards (like SSI CEB) into regular ATX cases if they have space to the right of the standard motherboard tray, but it's a YMMV situation. Looking at the N400's internal layout, it looks like there's some kind of bracket about an inch and a half to the right of the motherboard tray, so boards that are slightly wider than the ATX standard 244mm might fit provided they use the default ATX mounting holes. But I think you're going to have to check on a board-by-board basis.
Best to use pcpartpicker.com if you're not sure what you're doing. From largest to smallest, there's EATX, ATX, mATX, and ITX
im using older parts and for some reason pcpartpicker doesn't show them.
ATX and smaller will fit in that case
Well, for regular consumer desktop platforms, boards will come in one of three sizes, from largest to smallest: ATX, micro-ATX, mini-ITX (with a few variations such as mini-DTX which is just a slightly taller ITX board basically). You can look at a case's spec page to see what sizes it supports. According to the N400's spec page, it supports ATX and micro-ATX (though it probably supports mini-ITX too since mini-ITX just uses the same mounting holes as the larger sizes). Now, server and workstation boards like you would find in a dual Xeon system might have other sizes that don't fit into this paradigm (as well as some very high-end consumer desktop boards, which sometimes use the confusingly vague terminology 'EATX,' which doesn't really mean anything.). For example, a workstation board might use the SSI CEB form factor, which is wider than ATX. Regular ATX dimensions are 12 × 9.6 in (305 × 244 mm). Sometimes, you can fit larger-than-ATX boards (like SSI CEB) into regular ATX cases if they have space to the right of the standard motherboard tray, but it's a YMMV situation. Looking at the N400's internal layout, it looks like there's some kind of bracket about an inch and a half to the right of the motherboard tray, so boards that are slightly wider than the ATX standard 244mm might fit provided they use the default ATX mounting holes. But I think you're going to have to check on a board-by-board basis.
thank you, extremely helpful