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CSS block size is a box's extent in the dimension perpendicular to text flow within a line. It corresponds to physical height in horizontal writing and physical width in vertical writing. The block-size property expresses that dimension without fixing one physical axis in its name.^[ref][ref]

For an own example, block-size: 80px; inline-size: 200px; describes a $200$-by-$80$ CSS pixel rectangle in horizontal-tb and an $80$-by-$200$ rectangle in vertical-lr, under simple definite-size assumptions. The latter has block progression toward the right, but the dimension's magnitude remains eighty pixels.

Box edges matter. With box-sizing: content-box; block-size: 20px;, ten-pixel padding and two-pixel borders on both block sides give an outer block extent ofpixels. That outer extent is physical height in a horizontal writing mode and physical width in a vertical one. CSS box-sizing and the applicable constraints determine how the declared value relates to the content and border boxes.

The declaration height: 50px; block-size: 80px; also changes meaning with writing mode. In horizontal writing, both address physical height, so the later block-size wins when their other cascade priorities are equal. In vertical writing, height constrains the inline dimension while block-size constrains physical width; both values can apply independently.^[ref]

CSS block flow direction identifies the direction in which blocks progress, whereas block size is a measurement along that axis. Moving from vertical-rl to vertical-lr changes the progression direction without swapping block size back to physical height.

An automatic block size can depend on content and layout, while min/max sizing properties can constrain the result. The useful interpretation is therefore an axis-relative size with the box-model and layout assumptions stated, not a universal promise that one declaration always fixes the painted rectangle exactly.

CSS inline size is a box's extent parallel to text flow within a line. It corresponds to physical width in a horizontal CSS writing mode and physical height in a vertical writing mode. The inline-size property provides the corresponding flow-relative sizing control.^[ref][ref]

For an own definite-size example, let inline-size: 180px; block-size: 60px; apply to a simple box with no padding, borders or competing constraints. Under horizontal-tb, the physical rectangle is $180$ by $60$ CSS pixels. Under vertical-rl, it is $60$ by $180$. Inline size remains $180$ in both descriptions because the inline axis changes with the writing mode.

CSS box-sizing determines which box dimension a declared size constrains. For example, with box-sizing: border-box; inline-size: 80px;, ten-pixel padding and two-pixel borders on each inline side leavepixels of content extent along that axis, assuming no other sizing constraint intervenes. The outer inline extent is eighty, not fifty-six.

Logical and physical properties can address the same underlying dimension. With equal cascade priorities in horizontal writing, width: 100px; inline-size: 200px; supplies a later declaration for the same physical width, so the latter wins. In vertical writing, width and inline-size address different dimensions: physical width can be $100$ while physical height is $200$.^[ref]

A direction reversal alone does not change which physical axis is inline. Changing CSS inline direction from left-to-right to right-to-left can exchange inline-start and inline-end without converting a horizontal inline size into height. Likewise, CSS block size measures the perpendicular axis.

Real used sizes can also depend on min/max constraints, available space and the layout algorithm. State the writing mode and box-sizing convention before interpreting an inline-size value as a visible physical width.