A ConstraintHelper implementation that has additional layout params: screenHeight_percent and screenWidth_percent which allow to make referenced views height and width to be equal to screenHeight(or screenWidth)*some_percent_value. Where some_percent_value is a value between 0.0 and 1.0.

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="SizeHelper">
        <attr name="screenHeight_percent" format="float"/>
        <attr name="screenWidth_percent" format="float"/>
    </declare-styleable>
</resources>

SizeHelper.kt

class SizeHelper @JvmOverloads constructor(
    context: Context,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
) : ConstraintHelper(context, attrs, defStyleAttr) {

    private val screenHeight: Int = resources.displayMetrics.heightPixels
    private val screenWidth: Int = resources.displayMetrics.widthPixels
    
    private var layoutConstraintScreenHeightPercent = UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT
    private var layoutConstraintScreenWidthPercent = UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT

    init {
        attrs?.let { readAttributes(it) }
    }

    private fun readAttributes(attrs: AttributeSet) {
        val styledAttrs = context.obtainStyledAttributes(attrs, R.styleable.SizeHelper)

        layoutConstraintScreenHeightPercent = styledAttrs.getFloat(
                R.styleable.SizeHelper_screenHeight_percent, UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT)
        layoutConstraintScreenWidthPercent = styledAttrs.getFloat(
                R.styleable.SizeHelper_screenWidth_percent, UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT)

        styledAttrs.recycle()
    }

    override fun updatePostMeasure(container: ConstraintLayout) {
        for (i in 0 until this.mCount) {
            val id = this.mIds[i]
            val child = container.getViewById(id)
            val widget = container.getViewWidget(child)

            if (layoutConstraintScreenHeightPercent != UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT) {

                // layout_constraintScreenHeight_percent is set so override current child height
                val newHeight = screenHeight * layoutConstraintScreenHeightPercent
                widget.height = newHeight.toInt()
            }
            if (layoutConstraintScreenWidthPercent != UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT) {

                // layout_constraintScreenWidth_percent is set so override current child width
                val newWidth = screenWidth * layoutConstraintScreenWidthPercent
                widget.width = newWidth.toInt()
            }
        }
    }

    companion object {
        const val UNSPECIFIED_CONSTRAINT_SCREEN_PERCENT = -1.0f
    }
}

really thanks for this!!! @mzgreen